the Atlas is basically too slow (48 km/h or so) to be useful in mechwarrior 5 imo
Okay, there's other reasons why it's not good and the speed isn't the problem, it's more the lack of guns. You are significantly better off with the Stalker STK-3F with 4 medium lasers, 4 SRM6s, and 2 PPCs, and the Banshee BNC-3S with 2 PPCs, 1 AC10/LBX10, and fill the rest with medium lasers though I don't remember how many because I don't remember the vanilla MW5 mechlab hardpoint limits for that variant because vanilla MW5 mechlab is pretty trash. There's only a very small handful of mechs that can out pace the STK-3F and BNC-3S, but all of them either come way too late to actually matter, or you can only get one of them per playthrough.
without a risk of overheating
This isn't MW3, or MW4 where there's degrading effects for being at higher heat and the possibility of shutting down before filling the heat bar, MW5 uses MWO's heat system which means the only heat point that matters is the very last one. Also because there's only like maybe a handful of actually threatening units in vanilla MW5, shutting down from overheating is nowhere near as bad as it is in MWO or even previous titles. Besides, heat doesn't matter if the other guy is dead, and all the harder encounters in MW5 (both in vanilla and the DLCs) are burst DPS races.
It did play into that at the start, then there was a small problem that the setting made no sense when put under any scrutiny, a running theme that will persist throughout battletech's lore because very few writers understand how logistics work. And so because BT wanted to pride itself on being grounded sorta grimdark, the guys at FASA had to walk back a lot of the ideas of mechs being rare, introduce limited production of new mechs, but the circlejerk over the space roman empire (Star League/Terran Hegemony) remained.