Just noticed the extra damage is equal to the ability score, not the modifier. I thought it was lackluster, but that's pretty good.

I think the third benefit was probably ment for weapon or Unarmed attacks only, but with this, RAW, you can make an official muscle wizard.

its crap and let me explain why 

The designers at WOTC always overlook this one simple thing when it comes to magic item and feat design is that some subclasses crit on different numbers then just the 20 and this strips the fun of the feat from them and makes magic items weaker to give them bc it doesnt work fully with their subclass all bc they are so worried about the 20 instead of just saying "when you crit"

Why does this make it a terrible feat?

If WOTC wanted it to happen every time you crit they would just say that, but that would be insanely overpowered even more so than the other boons are.

This doesn't make any magic item or subclass "weaker", that doesn't even make sense to say.

it does make subclasses weaker as it doesnt take into consideration the subclass features that they made. Limiting it to a nat 20 limits it on the subclasses that do get bumps and increases to hit more crits. Why would it be a problem if the Champion got this at 19th level? the casters are casting wish and shaping the world this just lets that one subclass hit harder on crits which they hit more of

But that doesn't actually weaken that subclass, it detracts absolutely nothing from that subclass, it only adds to it. 

If this went off on every single crit, then every fight would just be a race to stun every enemy and have the fighter or monk go up and hit it with all of their attacks and be dealing 100+ damage per round.

A shame that this doesn't change for the one subclass and like two magic items that crit on a 19, but you're acting like every champion fighter is for some reason forced to take this feat and you're also acting like it's actively taking damage away from the class, which it isn't.

its about active synergy which this feat doesnt have 

It says that Bludgeoning, Piercing, and Slashing always ignore resistance, but I can’t tell whether that includes nonmagical resistance or not.

In the new Monster Manual, Magic Weapons no long overcome BPS resistance.  If it says they have BPS resistance, your damage is halved unless you have certain things, like this feat or a Vorpal Weapon.  But a lot of monster that use to have BPS resistance no longer do.

By your logic, a Champion shouldn't be wielding a Vorpal Weapon because it only does something on a 20.

makes sense to me they have an extended crit range give them a better weapon