I guess it just goes to show how different different people are, I hold basically the exact opposite view. Remember a few dozen numbers? Sure, no problem. Do some 2 digit mental math? Won't slow down the game at all. Use basic tactics? I'd be doing that anyways.
Come up with an appropriate, compelling consequence for "success at a cost" for roughly half of all die rolls? I'm going to struggle, and the quality of the game will struggle for it.
I could be misrepresenting a conversation I wasn't there for, but the OP made it sound like "Atlas" systems were systems that provide good sessions when run by good GMs and bad sessions when run by inexperienced/bad GMs. If a good GM runs a fast campaign with no pauses, and an inexperienced GM runs a similar campaign with a few 3 minute breaks to do some math or look up a rule, I'd still probably enjoy the second campaign, although likely not quite as mich. If a good GM runs a fun campaign with interesting and appropriate consequences, and an inexperienced GM runs a similar game with boring consequences that don't logically fit the set up, I don't think I would enjoy the second campaign much at all.
It's not about how "easy" or "hard" a system is necessarily, it's about how much the outcome depends on the skill and effort of the GM.