Too many crossovers spoil the plot.

People have this weird idea that suffering builds character and thus keep making their characters undergo endless suffering.

They usually complain about heroes getting 'handouts' saying that it makes things too easy for the heroes. Which i think is really hypocritical since these same people tend to love overpowered villains who pull out new plot powers every other chapter to perfectly counter every hard-won ability of the hero. Or, after allowing the hero to gain a boost of power, they immediately turn around and give the same empowerment to the villains to 'give the hero a challenge', even if there was no plot reason for the villains to gain said power in the first place.

They don't seem to realize that gaining new powers doesn't mean the hero doesn't need to train to use them, and doesn't mean the hero can't train to surpass the limits and abilities of their powers' previous users or create entirely new ones. Sadly, their limited imagination gives them the misconception that potential is defined by canon and once the hero overcomes all canon obstacles and reaches the canon ending, their path is 'complete'.