If GBA is outside a geonosian team, Hive Mind only applies to him and Brute. Hardly a "lead" ability - not substantially different than the Han/Chewie interaction or the Chirrut/Baze interaction. In a plug-and-play role, GBA is good, but he's essentially just a tank with a huge health pool, great self-healing ability, and good buff removal. He doesn't really do anything else. Not remotely game breaking on his own, just a really good support character.
If he's on Geos and not in the lead, you do get the full effect of Hive Mind. But there are lots of other characters which in essence allow a team to have 2 team leads. R2's stat boost and debuff cleansing. zBarriss unique. T3-M4's massive bonuses to droids. Traya's debuff cleanse. Rex's unique. Shaak Ti's unique. The entire Imperial Trooper mechanic.