I'm a season 4 veteran, I've picked Rek'Sai these days to see how her jungle pathing is and overall, how she feels again.
I've played around 10 games and I feel that there is some issues around her Ultimate.
Her Ultimate passive says you can mark a champion for 5 seconds after dealing damage to that enemy.
Her Ultiamte active makes you jump to a marked target.
I can't tell how many times I've hit her Q (Prey seeker) or simply auto attacked someone with her basic habilities in Unburrowed form, and even though it passes around 1 or 2 seconds of dealing damage, you can't ult exacly when you press the R.
I feel like you can't R whenever you mark a target and I've been testing it on normal games. Rek'Sai doesn't always use ultimate to a marked target exacly when you click it.
For what I've played, this makes her extremely unpredictable to the player using her.