My biggest problem with the design rules is that it is all too easy to become abstracted from the fluff and universe and just start to see the Mech or unit as a pile of numbers and stats with no correlation to the actual unit. After all, that is in essence what a unit in BT is, once you strip off all the fluff and fun parts, numbers. A Marauder can be refitted to be a Thunderbolt or a Guillotine, an Atlas can become a King Crab with ease. The only thing that stops this is the players keeping in line with the spirit of the universe.
I remember when I first discovered SSW and started to make my own units. They where all optimised with near max armour, DHS, high alpha and syenergy weapons, etc. I stopped because they felt boring and when I played them they where unfun. Later, after reading more and becoming more mature I went back and built units around an idea instead of the numbers and these worked out a lot better. Sure they did not have the optimisation but they where fun to play. I found a good rule of thumb is to have an idea when building a unit, or to stick to the theme of a unit that you are modifying. Are you working on a Marauder? Okay, remember it is built around a heavy energy battery in its arms and a backup left torso piece. Are you working on an Urbanmech? Keep it as a slow mobile turret.
In all honesty this hobby has the sad factor of atracting the minmax autists, and it is not just BT. When I played 40K there was a regular opponent of mine who used to bring the most munchkin army you could imagine - massed Librarians to get all the good phsycic powers, the biggest tanks he could get, etc. I used to let people bring customs to a game for fun but I stopped after he started bringing things that where basically pocket primarchs but at a far cheaper price, or units that could melee an entire platoon to death in a single turn. And when Formations came out he was the person who brought triple Vindicator units with libby conclaves hiding behind them, turning them invisible.
In the end people stopped playing him because he was never fun to face off against, you always knew that whatever he brought it was going to be as optimised as possible, and a one trick pony that was not fun to play against (Not that this or blatant cheating ever stopped him from losing, but that is tale for another time)
I guess my point here is that custom designs and forces work so long as the person making them has the 'spirit' of the game at heart, or at least in mind. Once you lose this and start seeing things as numbers or stats you start to head down the munchkin route and eventually end up fielding lances of Gausszilla's and the 7/11 laser batteries another poster mentioned, and then you become less and less fun to play against. In the worst case scenario this kills your community. So, always remember this when creating a unit, don't create the heat neutral massive alpha monster, create something fun like a siege Atlas or a Warcrimes Catapult with MG ears (Big Ish, if you are reading this that CPLT better be underway). Keep the spirit alive, thankfully BT does not seem to be as plagued by That Guy's and Timmy's as other games, so don't become one.
As for AC-2's, there use depends upon your field of battle. In an urban fight they are useless, but with a good field of fire they can be laying down a steady barrage of shots long before the other side is within effective range to reply, and through armour criticals are a thing. If you are playing in the right era AP ammunition is also a thing, and nothing beats lighting off the AC-20 ammunition in the other guys Atlas with your tiny 2 damage AC before he gets a chance to use it.
