While Druddigon has some valuable advantages, it is held back by many weaknesses, making it a low-tier Pokémon.
Druddigon has the defensive advantage of having Mold Breaker and Stealth Rock, and Glare and Toxic are useful status-inflicting moves. However, Druddigon's lack of recovery and meh bulk means that high-tier Pokémon could take it down with relative ease.
Druddigon's speed stat is absolutely horrible, and it probably won't outspeed anything. That being said, you will have to rely on Sucker Punch for moving first, and it can be countered by moves that don't inflict damage.
If Duddigon were to be in a higher tier, virtually any fairy or ice type would check it with ease.
There are simply better Pokémon out there that can set up rocks and inflict status.
I'm not at all saying that Druddigon is a bad Pokémon. It just isn't the best and it belongs in the NU tier. In fact, Druddigon is quite useful in NU. You mentioned it has helped you a lot in battles, so why not making a NU team and use Druddigon? It will do the job just fine.
Hope I helped. :)
