The 1990 sequel to Commando is a gigantic step up across the board. Playability, fun factor, presentation, everything is just better and more realized, and while Mercs (the collection only includes the Japanese version but I'm using the easier American name or I'll go insane) is definitely not a deep game, it's a lot of fun and I can see it being the bomb in coop with 3 players.
The story is pretty much this: three muscular dudes need to rescue the kidnapped U.S. president. That's it I think. Chaos and mass murder ensue. Mercs is basically a playable Schwarzenegger action romp, and is exactly as fun as that sounds. The skeleton of the game is the same as Commando: you run and shoot in eight directions. You can't aim, so you gotta go where you want to shoot, and that makes it quite hard; it's the one element that holds the game back, as I feel it would be greatly improved by just being a twin stick shooter. Regardless, you run through stages and murder a plethora of soldiers, vehicles and big bosses. Levels are definitely a bit more elaborate than Commando's, with different scenarios, situations and twists like small vehicle sections. There is a lot of stuff to destroy and lots of items to pick up, from health to points to new weapons (there are four if I'm not mistaken) to Mega Crush bombs to clear the screen and inflict heavy damage on all enemies. The game has great flow and never really stops you, with many popcorn enemies that get dangerous by sheer numbers and strategically placed beefy foes to act as temporary roadblocks. It's a really fun, smooth experience throughout. It's a shame that the final level is so anticlimactic, being a timed "boss fight" that doesn't really feel like a boss fight and that seems not really suited to single player at all (I could only clear it by spamming Mega Crushes, dying and spamming some more).
The presentation wraps everything with a neat little bow. Artistically, Mercs isn't terribly creative, being fairly grounded militaristic style, but bosses manage to impress and there is a good variety of mission locations. There are, however, many details that bring the game to life, such as visible damage on walls and buildings, enemies thrashing violently when ignited with the flamethrower, torpedoes soaring underwater with their satisfying trails and big underwater explosions, our hero getting blown back by the sheer force of the Mega Crush, and so on. Audio is on point and the music, while not wicked memorable, does a decent job in capturing the spirit of the militaristic action movies the game wants to imitate.
By this point, Capcom was mastering the art of polishing, and even games that are not particularly remarkable in theming managed to flex their muscles in the fun department. Power of experience and know-how, it's magical!