The only time that I like customizable loadouts is when they work on a credit system. For example, buying stuff during the match in CS, the old Tribes games, Shadowrun, NS2.
While I generally don’t like cross-match XP and unlocks, I suppose you could do a permanent loadout system that also works with credit system. Some abilities could simply cost more than others…Maybe every player has 100 credits, and you use 60 of those on a great gun, which means you can’t buy the more powerful abilities because you’d only have 40 left. Or vice versa.
What I don’t like is loadout systems that just give you slots and you put anything from a category into that slot. The balance constraints make it too boring. You end up having to tone down powerful abilities or weapons, and nobody uses the weak abilities. The beauty of a vending system in games like Starsiege Tribes, is that you can make the weapons and abilities as powerful or weak as you want…and then you just price them accordingly.
But the reason I still sort of prefer restricted loadouts with no customization is that you can see any player in the game and instantly know what abilities they have. I believe that visibility increases the strategy ceiling substantially… for instance, I dislike fog of war in RTS because I think it actually lowers the strategy ceiling.
So, I think that ideally:
Best: Restricted loadouts with no customization - The focus is on interesting combination of abilities, balancing strengths and weaknesses (just like a fighting game character, or an RTS faction)
Second Best: If there must be an XP and cross-match loadout system, limit the customization not by a number of slots, but by the costs of each ability. Every player has the same number of credits to spend on their loadout, and weapons/abilities are priced based on their perceived balance. The developers simply make the weapons and abilities so that they feel right…and they don’t have to throw out interesting abilities because they are too weak. “Balancing” merely becomes the process of changing prices for things…not the weapons or abilities themselves.