Im gonna say as a brief user of Thundurus, I think it may be broken.
To give you an idea of the power it has +2 LO Focus Blast does roughly 80% to 252/252 Bold Blissey, so you can imagine how many other things it can kill after a boost. That on its own is impressive, but its Fighting/Electric/Ice coverage is incredible, if you have any sort of entry hazard up for Shedinja that is completely unresisted. It could also pull a trump card and use Agility, however I find this ineffective as it is paper frail with an offensive EV spread and needs to OHKO everything. Besides specially defensive Unaware Quagsire, there isn't a single reliable switch for this thing, not that im downing Quaggy, I often used him for a physical wall. Well there is also Cresselia but +2 Thunder is a real wrecking force and you need more than just max HP to take that.
That being said, anything faster or something that can become faster, such as Azumarill w/Aqua Jet, Blaziken, and scarfmons all handle it if they get in safely. Its just the fact that there isn't a single pokemon out there (well, Shedinja) that Thundurus cant break through. Everything takes huge damage after a Nasty Plot besides that unless you're an Electric/Poison, Electric/Bug, or Electric/Psychic(so you resist Focus Blast/Thunder while being neutral to HP Ice). As far as i know Galvantula gets overpowered.
The other reason I think Thundurus deserves a closer look is because I suddenly started winning a lot more when I used him on a RD team. He destroys ANYTHING slower than him, even max HP latias get killed at +2. So while you can revenge it, you cant hold off its attacks at all so I believe it deserves consideration.
To give you an idea of the power it has +2 LO Focus Blast does roughly 80% to 252/252 Bold Blissey, so you can imagine how many other things it can kill after a boost. That on its own is impressive, but its Fighting/Electric/Ice coverage is incredible, if you have any sort of entry hazard up for Shedinja that is completely unresisted. It could also pull a trump card and use Agility, however I find this ineffective as it is paper frail with an offensive EV spread and needs to OHKO everything. Besides specially defensive Unaware Quagsire, there isn't a single reliable switch for this thing, not that im downing Quaggy, I often used him for a physical wall. Well there is also Cresselia but +2 Thunder is a real wrecking force and you need more than just max HP to take that.
That being said, anything faster or something that can become faster, such as Azumarill w/Aqua Jet, Blaziken, and scarfmons all handle it if they get in safely. Its just the fact that there isn't a single pokemon out there (well, Shedinja) that Thundurus cant break through. Everything takes huge damage after a Nasty Plot besides that unless you're an Electric/Poison, Electric/Bug, or Electric/Psychic(so you resist Focus Blast/Thunder while being neutral to HP Ice). As far as i know Galvantula gets overpowered.
The other reason I think Thundurus deserves a closer look is because I suddenly started winning a lot more when I used him on a RD team. He destroys ANYTHING slower than him, even max HP latias get killed at +2. So while you can revenge it, you cant hold off its attacks at all so I believe it deserves consideration.