A proper main battle tank gives you a good, reasonably balanced combination of protection, mobility, and firepower. (That's why I'm only half-kidding when I claim on occasion that the "real" MBTs of the BattleTech universe are the 'Mechs, especially those in the medium and heavy weight classes -- they're generally both tougher and better at handling terrain than tonnage-equivalent tanks and don't skimp on the guns, either.) Anything going 3/5 or less, explicitly including such luminaries as the Alacorn and the Gürteltier, automatically disqualifies itself on the "mobility" front; it may well make a great siege gun and/or defense turret and be generally scary to encounter, don't get me wrong on that, but it's just not an MBT.
Picking an artillery piece for its main weapon only compounds the problem. BattleTech artillery isn't generally accurate enough to put on platforms whose job it is to roll out and meet the enemy on purpose -- while you're still trying to land a shot in a hex close enough to the enemy unit this turn to do some damage, it'll use its own weapons back on you and in all likelihood hit a fair bit more often. Worse, even when you do get a shell right on target...the result may well fall a good bit short of "impressive". A Long Tom HE shot, about the biggest you're likely to get, deals 25 damage in 5-point clusters to everything in its target hex; unless a unit there is one of the types that take extra damage from area attacks, that's not really worse than taking a solid whack from a couple of decently sizes LRM launchers that turn instead, something many 'Mechs and even quite a few tanks can weather easily enough.
So what these really are are self-propelled artillery pieces with extra-heavy armor and some self-defense weapons just in case. Siege guns, as I said. Siege guns that are arguably trying to do too much all by themselves at that -- if they're using their artillery from a proper stand-off distance, they won't need the secondary guns and all that armor plating much, and if they ever actually do find themselves in the thick of things where those might become useful, then the tonnage invested in their heavy artillery piece is largely going to waste. They could really shed some weight by making up their minds on just what it is that they want to be and then properly focusing on that role.
