Degthai knows the dishes. The problem is a kitchen that has calibrated everything toward inoffensiveness — curries that arrive pleasant rather than complex, noodle dishes that function without exciting, heat levels set for a crowd that might complain rather than one that came to eat Thai food.
It's not aggressively bad. It's just consistently below the standard Nashville Thai has reached. SS Gai is doing something genuinely interesting a short drive away. Degthai is doing something familiar a short drive from wherever you are.
The flavors are recognizable, the curries are inoffensive, and nothing arrives with the heat or depth that makes Thai food worth eating. Nashville has better.