Yeah, Teemo was always reviled on SR and has been one of the overpowered/goto champs since the new TT came out. I don't doubt BFT is making things worse, but I think it's making an already good champ even better more than just being 100% stupid in its own right.
That said, I've never had the pleasure of seeing it in action, so I wouldn't know how much it's helping or hurting specifically.
Well, I haven't seen anyone mention the Mpen aspect or the CDR from Blackfire Torch, so it really is a horrifying item for Teemo. He can just get Sorc. Shoes and BFT for 40 Mpen, and God forbid one of your team having not bought any MR at all by that time. Seriously, it starts coming out to his shrooms hurting almost more than he does overall. One of the last Teemos I fought had the most kills in his team, but was 2 levels behind just because he sat around the jungle.
Yeah, some champions use BFT more effectively like Mundo, but none of them can just walk around your jungle laying shrooms and slowly wearing you down from across the map with minimal effort and complete safety since it reveals your location, too. It's bullshit. Complete and utter bullshit that makes me rage. At the very least, Karthus ultimate is obvious. With Teemo, just running into the jungle is like playing a game of Russian roulette.
Personally, I've had the pleasure of meeting him multiple times on TT. I really should get Nocturne and just spellshield all the goddamn shrooms.