I've been looking for a custom faction to fill the last AI slot in Void Wanderers lately, but I've noticed a problem in that most of them seem grossly overpowered in comparison to the stock factions.

The only one I've found so far that fits in among the original factions is OmegaDrace's Sergal Faction.

I haven't gotten around to testing all of them yet, but are any that I've missed that are still 1.05 compatible?

I'd probaby try MDC, General Industries and maybe Untitled (you can find links in Void Wanderers OP). You can also take a look at Pelian Army, but you'll have to make your own faction file for them.

Whilst it's safe to say that the various Space Marine chapters in the 40K Recomp are utterly unbalanced to stock/vanilla, I'd recommend trying either of the Imperial Guard options; they're much more 'stock balanced' now.

Finding Imperium tech in the little loot boxes can be fun too, it'll give you some fun toys to play with without destroying game balance too badly.

The stock option with laser weapons will be somewhat harder to face off against than the autoweapons, as beams have no drop and instant travel. Fair warning.

Balanced faction in comparaision to vanilla factions are rare because it seems The vanilla factions uses .177Pellet Ammunitions. (A.K.A I kill you with a full-35 magazine.)