Aussalt space Marines not o.p., but the reason they feel like they are is because units like them (the storm boys for the orcs and warp spiders for the eldar) are locked behind a elite wall and you only get spawn one of them. So in most cases a space Marine player will be able to get out a few ASM and get them upgraded with power swords before a player playing a different race can get there elites out and if you don't pick warp spiders or storm boys as a elite you are giving up your ability to harass control points and player units, which will put you at a severe disadvantage when playing against a space Marines problems. Yes ASM need a little balancing, but that isn't the core problem
The core problem lies within the games elite system and to a lesser extant the doctrine system. The elite system I feel is a big problem because what exactly qualifies a unit to be a elite unit why are storm boys and warp solders elites and not ASM, the other problem with this is that it makes the elites impossible to really counter unless with a nother elite which creates very boring gameplay in my opinion.
So my proposal to fix this is exchange the eilte system for a hero system if you will, so put every unit back as base units like warp spiders, storm boys, terminators, ect. keep the heroes In the god tier or whatever you want to call it like Gabriel and maka. Then make the heroes cost either power or req, or maybe so of both, this way you have to make a choice do you want Gabriel or do you want terminators, ect. You could even keep the heroes as one man armies if you wanted to, but I would suggest boosting ever units health across-the-board, because die way to fast In this game, maybe give some more defense.
My heroes system would solve some big issues this game has. With system you could have counters to hero units like terminators, it would also take away from some of the to much mirco-managing this game suffers from. They could also give basic units like tactical Marines more upgrades that could be able to counter heroes. This would create more depth of gameplay, because this game seems pretty shallow in Terms of strategy.