Effect can only activate when Flip Summoned which is very slow.
Weak attack stat.
The problem with Subterror decks is that all Subterror Behemoths are high-leveled monsters, so getting your first flip monster on the field to start Special Summoning can be a problem.
Golem Sentry works well in a Subterror deck by letting the bigger monsters trigger their Special Summoning Effects. He even has a similar playstyle to Subterrors by flipping himself down to reuse his effect.
Dude, if you struggle to farm legendary duelists at the gate... it means that the problem is yours. You need to get used to these cards with many effects... remember that the DESTINY of duel links is to reach Arc-V and Vrains world.