The only reason Doryuuzu seems "nowhere near broken" is because his most popular set is suboptimal and doesn't fully utilize the amount of sweeping power he has at his disposal.
Jolly Baloon Dory has a couple of perks, such as Spikes immunity, and setting up on EQ walls without a secondary attack, but it also strips him away of any hope he has of getting past his his usual counters, like Gliscor, who can be taken care of with an Adamant +2 LO Rock Slide flinch and subsequent Return, or Breloom, who can be KOed on the switch-in with Return and minimal prior damage (like one turn of LO recoil). You can also 2HKO Skarm with +2 Rock Slide, but such is not the case with Jolly Balloon.
Also, LO sets give him some breathing room to switch in on resists, whereas with Balloon, you have to play extremely conservatively. With most players, he's effectively MIA until the very last turn or something really fast needs to die.