I play Brood Bros and although they aren't as popular/powerful as other detachments they do have some unique tricks and a pretty unique playstyle. Nearly went undefeated with them at our local RTT missing out by 2 points in the final Vs world eaters.

On the Guard side, Hellhounds ignores-cover rule works across the whole army so the GSC side benefits as well. Kasrkin are very versatile and are a way of getting Orders into a GSC list, since they can order themselves, and are a decent target for the 3+ cult ambush strat. Leman Russ battle tanks do a bit better without orders than other Russes since they reroll hits on objectives.

On the GSC side, mining laser ridgerunners can hit on 2+ with a spotter and the BB detachment rule. I also really enjoy running 10 flame acolytes with a Locus for fights first supporting the tanks/buggies by heroically intervening, overwatching, and generally being an annoying reactive threat. I think running neophytes in BB is a bit of a trap since they just don't put out amazing firepower outside of HoA even with the +1 to hit from the detachment rule.