I tested the Standard OU set of this pokemon, which has Cosmic Power, few time ago, and you are overrating this 'mon, it has nothing to do against most Offensive Pokemon with Substitute since Stored Power without boosts is too weak. Taunt foes cripple it and it can be phazed or Perish Song'd
It has nothing to do against Fire type foes and receives too much damage from Strong special attackers.
It has troubles against Toxic Orb Gliscor, so, most of them either SD or Taunt versions.
I am not saying this 'mon is useless in OU, by any means, but it is not close to invincible even if it is the last 'mon of the team.
Burning opponents while inmune to status, SR and weather is funny though.
A few points to be made.
Firstly, offensive pokemon with substitute rarely have taunt or boosting moves, so they're straight up setup bait. Get up to plus six and i can tell you that you'll be breaking those substitutes. Phazing means you have to get burned, although it's definitely doable. Perish song ditto. Taunt messes him up, but it is possible to outspeed and burn. As for toxic orb gliscor, only taunt versions are a huge deal. Otherwise, just keep boosting and then spam stored power. As for fire-type foes, let me break down common OU fire-types for you.
Heatran:As shown below, heatran can roar it and not particularly care. If it lacks roar, however, it's done without crits.
Infernape: Doesn't want to switch in on even plus zero assist power, and isn't really powerful enough to not be setup bait on its mixed set either, in my opinion.
Ninetales: You MUST be joking. Still setup bait.I suppose a specs set might be a little threatening, but i'm not sure, and those are pretty rare regardless.
Darminitan:The main problem. Of course, since he's probably unleashing flare blitz, you can take one and roost. With that and SR up, once he switches back in he's nearly dead anyways.
As you can see, common OU fire-types fail against sigilyph, other than arguably Darminitan.
The main way i deal with it is roar plus protect heatran, and sableye. Heatran roars it away, taking only 12% from the plus one stored power, which it can protect off with lefties. When it's the last pokemon, i can sacrifice something, then bring in sableye, then finish it off with taunt and night shade.