Gothitelle is particularly good at walling, so you don't really have to worry about dying from strong Bug-type/Ghost-type/Dark-type attacks if you a high defense stat (which Gothitelle is all about).
And since her speed is mid-high, she can outspeed some of the mid-level Darks and Bugs in the metagame (lol, though, I did EV train mines in Speed a bit). If you can do that, have her use Charm on the Dark types, which lowers the ATK stat by two stages. Since, y'know, most Dark-type moves are physical.
By saying that Gothitelle is worse than Reuniclus just makes Gothitelle look even worse. Sure, Reuniclus has that amazing Sp. ATK stat, but it has sub-par defenses and is literally as slow as a rock. Even then, there's the fact that Gothitelle can learn Dark Pulse, and her moveset is designed around literally degrading the foe (Fake Tears, Embargo, Flatter, Charm...), then going in for the hit.
In general, the odds can be turned on each other, but it's not like either one can learn Bug Buzz.