Isn't a big part of the risk of Fatal Blows is that it's a one use only option? Use it in the first round and it's gone for the 2nd and 3rd if you get that far. If you keep it for the 2nd you might be down 1 round and gives the other player some momentum to pummel you in the 2nd round.Its just like Xray in MKX. But at least the Xray had risk reward balance.
It definitely adds another layer of mindgames to deal with.
But at least in MK11 you still have your def/off meters to play with so you're not completely stuck with no options. And seeing as it's only ~30% damage, it's still not much worse than many combo's esp. when a KB is added.
Some should probably be tweaked as they're a little too OP as have been mentioned, but overall they're not an "I win" button as it seems many as saying.