Only potential problem I see is Tentacruel in rain, since Scald 2HKO's Gengar (which may not be a bad thing if they're not expecting DBond) - but if Cruel decides to Protect, which it may as well since after one Scald Gengar usually won't be in range to sub, you're in trouble. Weavile can't really trap Cruel either.
Because of this, and since rain is everywhere, perhaps Thunder is warranted somewhere. If you choose to drop FB, Tar can be handled by double switching to Weavile, and if Gar's done killing the spinner it's mission accomplished anyway. Running Magnezone also allows you to forgo HP Fire since you can trap steels in a similar fashion.
I'm usually more worried about Gengar being setup bait more than anything else, and Thunder allows you to nail the likes of Sash Cloyster and Genesect with a 30% chance paralysing, which means Gar scores another kill.
Since I play it with Deoxys-D, usually Tentacruel shows up at start, and Gengar is in range to substitute after a Scald.
What I want to point, is that it's very likely that Tentacruel will be full life, thus there is no reason to Protect to get lefties.
After all the games I played there is only ONE Tentacruel that did not Scald when Gengar was in KO range. I could bet that he knew my RMT by the way.
The fact is that usually they are afraid of the Sub. They can Protect to scout the Pain Split, but it has no sense, since if they predict sub/split set, they won't let Gengar have this sub.
Ofc with a Burn, everthing is different.
But usually Rain teams start Politoed then Tenta, or straight Tenta, and I play around with Deoxys-D by keeping setting hazards to have an idea on how they predict Gengar.
I didn't want to say that Tentacruel is easy, since I rely on Destiny Bond, but I bet on the surprise factor, it always worked fine.
Sometimes I also just do spam Shadow Ball on Tentacruel, so once Gengar dies, I threaten Tentacruel with another sweeper. It works on HO teams, since Tentacruel will have no room to come in and out just to spin given the hazards are the sweepers around. It can seem totally silly to lose 3 pokemons for this matter, but in the meta with Hazards up, there is really few room to stop sweepers like Scarf Keldeo or RP Genesect etc.
It's not a 100% accurace strategy (and I don't want to make it look like that, what you say is relevant), but it sure does work a lot of times.
So yes, it can seem like I avoid the issue, but from the games I played, I never felt Thunder needed. And well, Thunder doesn't kill Tentacruel either, if you have been burned, he will stall you with Protect as he would have done with the other moves. It depends on the team in my opinion, Thunder could be a superior choice if your team can't force a Tentacruel out at ~70%. But anyway, against a Rain, they let your Genesect set RP and you're in a rather good situation, no matter your hazards.
*Against Genesect you HP Fire, Cloyster you Focus Blast, and Revenge kill it with any Priority, but anyway Hazards should be up so this shouldn't be an issue ;)