in my opinion the truth is in the middle: ac20 is gigantic, weights just too much, has only spare ammo and pitiful range, but no other weapon can replicate the joy of ripping arms and legs and *heads* in a single shot. when an ac20 is around any minimal mistake means death, even if it sucks everybody play with the ass clenched.
after all, we all know no weapon is better than the medium laser.
If you want to rip things apart, a pack of 3 SRM-6, a PPC and an extra heatsink will serve you better in the place of the AC20 in the Atlas.
The reason why a AC20 is the line between a moron and a veteran is the range (and note I said Battletech, not MW). With an extremely limited amount of ammo, you want to make every shot count. That means you don't want to shoot at everything every time. The problem is that with the typical speed of 'mechs it is mounted on, getting close enough to get that shot is practically impossible. Just about anything will be able to out manouevre it. You will always be better off replacing the AC20 with other things (e.g., LRM20 or PPC with extra heat sinks, etc.). An AC20 on a 3/5 'mech is a joke.
Even in an urban environment, where the AC20 is supposed to shine, replacing the AC20 with a trio of SRM6 will give you the same average damage (and higher potential damage) and save you 8 tons of weight. Replacing it with 4 SRM6 (12t vs 14t, 8 crits vs 10) will actually give you much more average and potential damage although your ammo per ton will be slightly less. With the Atlas, you have the advantage of being able to share the SRM ammo that it already has. Of course, the heat will be higher, but that is manageable, especially with the 3 pack rather than the 4.
Autocannons have long been acknowledged as being the most inefficient weapons in the CBT universe, which is, of course, the running joke of Davions being enamoured with the stupid things (and central to the Jaegermech story). There is no AC that is worth the weight in the 3025 era. LB10-X redeems the category somewhat, but I would still prefer a pack of 5xStreak SRM4 over the LB20-X.
Even if the combat is in real time, anyone who has played The Crescent Hawk's Revenge will tell you the Archer and the Catapult is far more dangerous than the Atlas. The Puma is scarier than the Black Hawk for the same reason. And the best 'mech you can get in that game is not the Atlas but the Marauder*. The reasons are exactly the same: Long range firepower.