While the super speed, strength, and fun abilities like gliding could be worth it, the big problem with Mercer's powers is they're very destructive. His footsteps leave cracks on the ground, and such. Using your powers will inevitably accrue a lot of property damage. And while you could easily shrug off anyone trying to make you pay for it thanks to your immortality, it's still kind of a jerk move to go around breaking everything. Pretty much everything else in his toolkit is either born from or made to murder. His superhuman healing, memory assimilation, and shapeshifting all require killing people, and all of his shapeshifting weapons don't have a ton of use outside of, again, murder. Which, like, if you're okay with the mass amounts of killing you have to do to take advantage of these powers, that's a little concerning. Though, you could theoretically find other uses for his weapons, like, maybe construction/demolition? Plus, Heller's version of the powers are cooler.