They have Lv. 4-5 Shields on virtually all their ships, and almost no ships in the game have Piercing, meaning landing a shot on them is extremely difficult. Their main guns are heavier than the main guns i use on the ships i design personally, and the ACAC they have is also quite impressive, so the use of fighters against them is also very limited. The Kalaedinese might be able to go toe to toe with them, but the human forces seem like they would be completely trashed in an even fight.
Apologies if it is bad form to necro this thread, but I can confirm that Negali are more than a match for anything in the ISS. I've fought them in my last four Starmada battles, losing very badly each time. In one loss, they scored 10x as many victory points as me playing Imperials/S'ssk.
Negali have decent speed, great shields, and incredible weapons. If anyone has suggestions on counter designs, I'd be open to hearing them. I've no idea how to begin, as they seem to have no apparent weakness (I'd say it was long-range, if their Shock Cannons were RNG 15).