You do know that leech seed also restores 12% of the opponent's health no matter what, right?
Meaning leech seed is bugged in two ways (one of them being the accuracy bug)
And yes, as Rekkuza said, the exclusion of leech seed isn't really a team killer
Ferrothorn and Tangrowth - yes leech seed is indeed an essential move for those two.
However, I think they can manage without the said move
Ferrothorn: Spikes, T-wave, Gyro Ball, Power Whip
Tangrowth: Knock off, Giga Drain, Sleep powder, Ancient power/Hidden power
That's just my opinion
Also,
I do PVP in PRO