elite dark slayer
Never having watched Dragon Ball Z, I can't really take part in that particular argument, but let us just try and imagine the consequences of giving a main character an incredibly powerful skill that can theoretically be used at the drop of a hat.
Narratively speaking, this would mean that you have written yourself into a corner, giving the character an enemy who cannot be defeated normally. Now, you have four main options which immediately present themselves. Firstly, you could go with the 'the character unlocks a super-powerful ability' route. Secondly, you could go with the 'the hero gets help from an outside source' route. Thirdly, if you have had the forethought to make your character one who has proven to be resourceful and intelligent in the past, you can have your character use this to get out of the situation. And fourthly, you could let the character lose. Okay, let's see what happens with each of these options.
For the first option, if you want your audience to accept it, it will need some explanation before or after the event. If you do it well enough, loose ends from some time before the 'transformation' will be tied up by this, giving the audience at least something to hold up their disbelief with. Even after you do this explanation, the question then becomes 'what now?'. Now that the character can do this, will it become a recurring thing, or will it be explained it such a way that the character can only do it once, or only under a specific set of circumstances? If it becomes a recurring ability, then your antagonists will have to become stronger in order for any real conflict to occur, because conflict, in some form or another, is what drives a story. This will eventually end up in a fairly ridiculous powercreep occurring, something that I gather is happening in Dragon Ball Z. If the character can only perform this ability once, then yeah, maybe it can be acceptable, if you have a good explanation, I won't deny that. No real downsides, except for the idea that some of your audience might take a step back to think, and come up with the idea that it is incredibly contrived unless you foreshadow it for very long beforehand. To carry on the analogy to a television series, you cannot foreshadow such an ability at the start of an episode and then use the ability just later on in the episode. It would need to be at least mentioned at the start of the season, with the ability being used near the end of the season, or even mentioned several seasons before. This shows your audience that you have given the ability some thought, and not just thrown it in to save your narrative. It could even prove suspenseful, making the reader wonder 'when will the character use the ability?', especially if it comes with significant drawbacks. Now, if the ability can only be used under a specific set of circumstances, the result depends on how specific your circumstances are. Not specific enough, and you end up with powercreep, a la the recurring ability. Too specific, and it looks contrived again, ruining the suspension of disbelief. There should theoretically be a sweet spot between the two, but it is very hard to pull off, and would still probably need some mentioning beforehand. Way beforehand.
Now, the second option. The character gets help from an outside force. This could be the character's friends, or someone else whose motivations happen to involve helping the character. If it is the character's friends or allies, this will require some setup beforehand, explaining why they are helping, how they knew to help, and why they were even around. One example of this done well would be Gandalf arriving with what is literally the cavalry during the battle of Helm's Deep. Where the cavalry came from, why Gandalf was initially missing, and why he came back at that particular time were all fairly well explained, leaving the audience in no doubt that while this was planned by the author, it was in no way contrived. Once all these conditions are successfully met, the plot can continue as normal. If it is not an ally, there are three options: a new character, a previously neutral character or a different enemy. A new character can give the plot a new element or direction, or can explain certain occurrences which have previously taken place. Both are satisfying conclusions, either driving the plot or tying up loose ends. But the new character must show up again, or it starts to look contrived. A previously neutral character will drive the plot, too, especially if you explain why they now side with your character. Also a satisfying conclusion if done right, and would lead to character development for this former neutral party. This leaves the option of the enemy helping the character. This one's an interesting one. The enemy is either performing a heel-face turn, which can be difficult to write believably, or his motivations just so happen to line up with the hero being left alive. A heel-face turn can be an interesting way to resolve conflict, or it can be boring and unbelievable. Depends on how you end up writing it. If the enemy's motivations involve helping the character, expect some interesting conflict and distrust between the character and his enemy during their temporary truce against what is their mutual foe. These motivations could be a new, nefarious plan, or just 'teaming up against a bigger threat'. This also has the effect of giving you a perfect opportunity to either shine a light on the enemy's motivations, or foreshadowing a new plan on the enemy's part, giving you a brand new story arc. All of these are fairly acceptable solutions, rather than giving your character a deus ex machina ability.
Now, one of the most fun options, in my opinion. The character thinks his or her way out of the situation. This gives a new dimension to the plot, showing that your character does more than just whack foes until everything becomes better. No, instead, your character is one who thinks. Anyone can be a brainless but powerful warrior, but one who shows cunning and resourcefulness is character who is not guaranteed to always win, but will go far in achieving their goals. This prevents powercreep, especially if the character is not amazingly powerful in their own right. It also provides suspense in future battles, as the audience can never be absolutely sure that the character will win, especially if you show that you are willing to let your character lose once in a while. It keeps things believable, makes the character relatable, and keeps conflict at a manageable level. A resounding success all around.
Option four. One of the options many writers are hesitant to try. Let the character lose. If the character is not the protagonist, the character might end up actually dying. This drives the plot, and may make the enemy even more despicable in the eyes of the audience if the character is is beloved by the audience. Think 'the death of Obi-Wan Kenobi'. If the character is the protagonist, the protagonist is usually protected by plot armour and either survives somehow or dies, but doesn't die completely. He or she survives as a ghost or something. But the death of the protagonist doesn't happen often, unless it is to make way for a new protagonist. But, again, as I said, plot armour and many writers being sentimental about their characters, so it won't happen often unless you're reading Game of Thrones. If the character survives, they have the opportunity to come back stronger, coming back knowing the enemy's weakness, or coming back with help. If they come back stronger, you have your first option again. Powercreep ensues. If they come back knowing the enemy's weakness, you have the third option: resourceful character that the audience can root for and relate to! Especially since the character failed at first, something I'm sure many have gone through. If they come back with help, that's your second option again.
Sorry to make you guys read all that, do tell me if I missed anything important. But if there's on one thing I want all of the 'TL;DR' people to take from this, it's this: every time you write something, you must consider the ramifications on the plot and how your audience views it. Especially with something as gamechanging as giving your character a superpower.