Nah I don't think they're overpowered. Things like Latios can easily be revenge killed by something like Scizor, or Gengar if it has HP fire, Dragonite is easily killed by Weavile or Mamoswine with Ice Shard if SR is up, and so on. Basically, they all fall to steel types and priority. ;x Just lock them into an outrage and you're all good. Plus, continuing, Hydreigon can be killed by Terrakion and boosted Scizor (iirc), Flygon can be 2HKO'd be Scizor I'm pretty sure, Altaria is lol, Salamence - steel, etc. Plus they're all (except the gen 4 steel/dragon legendary) weak to status aka Toxic. Just stall and you're all good imo.
I'd honestly say certain other Pokemon which are found in OU are more overpowered ;x PLUS, other Pokemon have higher attack power, for example, for special... Alakazam and Volcarona all have a higher sPA stat than Latios - the highest dragon. While for attack, although Haxorus is the highest, Conkeldurr and Metagross are still high up there in attack. Oh yeah, and Jolteon, Dugtrio, Alakazam, Starmie and Tornadus all outspeed Latios, the next in line in terms of speed. As for ubers, in terms of attack power, Deoxys-A still has more power than Rayquaza, and Deoxys, Excadrill, Groudon and Ho-Oh, as well as Arceus are in the top 10. As for sA, Deoxys-A, Mewtwo and Kyogre have the strongest power in sPA, ahead of dragons. In speed, most things outspeed the dragons in ubers.
...but yeah I'd say there are more overpowered things than dragons as a whole, it just depends on the dragon in general I'd say.