This card is sort of the classic Aggression card, but in a negative way, it kind of exemplifies how the designers don't like to give Aggression good cards. 5 damage for 4 effective resources is just not good enough, you should be getting 1.5 damage per resource. But the problems with this card go beyond mere efficiency. It is an expensive card with fairly high damage, which is not particularly good as it tends to overkill some of the minions, you would often rather have cheaper cards that do damage in smaller increments. More significant than that is that the cost of 3 is high enough to reach the point where playing a cost 3 ally would be a more efficient way of dealing damage. But the most disappointing thing about this card is that even if you do make a point of putting it in your deck and not putting in any better alternatives, and you get into a scenario where you really do need to deal damage, it can still end up sitting in your hand because most heroes have a fair number of hero-specific attack cards, and designers are not shy about making hero-specific cards which really are very efficient and far better than Uppercut, so you would rather be playing them.

Uppercut isn’t useless, sometimes you draw it when you need to deal damage and don't have a better card. And it kind of comes into its own at the very end of the game when all you care about is dealing damage and you are happy to find any card in your hand that focuses purely on dealing damage. It is just a disappointing card that I stopped using once I got enough expansions to have more choice in what cards to put in the deck.

Combat training is one of these cards whose efficiency is kind of doubtful but I like using it anyway. Compared to other standard permanents it really isn't very good. It only costs one less than a Helicarrier, but a point of damage is much less useful than a resource, and it only gives you that point of damage on turns where you use your basic action to attack, meaning it does not work on turns where you thwart, or defend, or recover, or use an action to remove an attachment, or are exhausted by an encounter card, or exhaust to remove an obligation or perform any other effect. That totals up to quite a lot of turns where you can't use it. Some characters have ways to attack more than once per turn and thus gain more than one damage per turn from this card, but except in the very strongest cases this doesn't really happen often enough to fully overcome all the turns where you can't use it. And characters who are extremely good at making multiple attacks per turn also tend to have high thwart values that make you want to double thwart instead of double attack.

Nevertheless, having another power up permanent in your deck can be handy, and it is just fun to permanently improve one of your combat statistics and permanently become a more powerful attacker. And you could be casual in when you play it, ignoring it if you don't have the resources for it, or playing it when you don't need damage so that you have more damage when you do need it, or maybe you can wait to play it cheaply with Power of Aggression.

Generally you want to give this card to a character who never thwarts or defends, only attacks. And having a few cards that let you double attack, like One-Two punch, is a nice plus. Or you can give it to a character like Captain America who usually thwarts but has a decent attack and is extremely good at making multiple attacks per turn, just because it is fun to give him a balanced 3 ATK and 2 THW and give him more interesting tactical flexibility.

Obviously great with Drax who needs to get hit a lot. But it also works very well with Star-Lord since his allies always have the GUARDIAN trait, making them very efficient blockers. And it can be a great combo with the Groot basic ally, since he can block an attack from a minion, get tough, heal and in the next turn block an attack from the villian and heal once more.

How does this card interact with Rocket Raccoon'salvage ? I guess we can apply both effects, in any order you wish, right ? (in this case, it's OP)

If ever there was a card to be Errata'd, this would definitely qualify. You're spending 4 resources to effectively deal 12 damage. Sure, it's restricted to the boss - but that's your win-condition, so not really that big of a deal. That's a 3:1 damage ratio on a Basic ally card that literally every Hero can afford except an early game Iron Man. If you're playing a Guardian Hero, it's hard to pass this card up just because of its hyper-efficiency.

Also worth noting; this essentially makes a card like First Aid a 2 resource 6 damage attack.