Recently tried a Diable playthrough with Nexelrin, great looking mod, but it felt like they might be a little too well-balanced. Their home system immediately got a Persean League grand invasion, and they just could not stop it, the only fights they ever seemed to win were when they had an overwhelming numbers advantage, in anything resembling a straight fight the Diable fleets just got annihilated. The Cronus-class light battlecruisers from Dark.Revenant's ship pack, in particular, were a problem (I realize they're from a mod and quite good, but given their light loadout for a battlecruiser and use of an ability you see on ships from vanilla, they seem in line with vanilla for a high-tech ship at least, and the rest of the Persean fleets seemed to be vanilla, more or less), one of them could engage two of the standard Diable battlecruisers (forget the name at the moment, sorry) and win without a scratch. Diable's strength seems intended to be in their fighter wings, but if they're evenly matched or outnumbered, high-tech ships just rip the wanzer wings to pieces with massed beams before they can accomplish much. This basically means Diable's lost, because their battlecruisers have middling armor, laughable shield arcs, mediocre firepower, and their ship system basically just gets them a brief instant of speed which is not nearly enough to disengage from high or mid-tech opponents to replenish their wings.
I loaded saves many times to see if I could salvage things but no matter what I tried it was just never enough. It could just be that the ai fleets were terrible at picking deployment compositions and playing the faction in general. In which case I suppose there's not much to be done, though I might suggest considering changing the battlecruisers ability to something to support their wanzer wings, like maybe a temporary shield or damage mitigation. Something to help the ai keep their fighters alive even if they're using them poorly. Also maybe upping the armor or hull a bit more if they're going to be that slow and poorly shielded.
Either way, great mod, I really liked what I got to see of the smaller ships before we got pounded into the dirt, it's always nice to see a faction which isn't just super fine-tuned and upscaled vanilla concepts with a fresh coat of paint. ( To be clear, not badmouthing mods like that, vanilla concepts with a fresh take and coat of paint can also be nice and good.)