So I just played against a Singed in URF....
It was the most toxic experience ever. I mean yeah, I killed him every chance I got, including before he even started proxy farming(just so happened to be in the base when he came in), and he just kept TPing back to base and farming ALL of the minions. I couldn't go to lane because he revived so fast, and he could proxy farm all day because he had unlimited mana. The game wasn't FUN whatsoever because I was playing Singed's game(like you always do with this boring to fight against champion) rather than my own. We of course lost because they were able to outdo the rest of my team and I tried a similar strategy to even it out but because I built squishy starting off and hadn't enough armor the Fiora just came came to base, used two Qs and killed me. She was always around there despite when I would come in at random times thinking I'd be safe for at least a few waves. Why is Singed allowed in a mode meant for fun whereas he can proxy farm infinitely making the game the most passive thing ever(Well I say passive, because it doesn't feel like killing him is worth anything since his goal is to be a nuisance until he is eventually killed. Singed basically ignores you and you just kill him despite it being no fun at all, which is why I call it passive). I mean I made a post about Veigar being broken in URF, and I think he is pretty broken, but Singed while may not be broken(he may also be, I haven't had enough games against them to know if this strategy can be countered with infinite mana and not even having to build mana items), he's definitely no fun to deal with in URF where you actually WANT to fight people.
WOULD LIKE A WORD WITH YOU
. The reason swain is good is because he can stack his Q and use W and E often. Singed and fiddlesticks can't do that with their abilities. Singed presses Q and that is that. Fiddlesticks drains and just keeps on draining. If he activates again it just turns off even though the cooldown is already done.
I don't think singed was the biggest problem.