Rek'sai my thoughts (for whatever its worth)
So fair warning, I've only played 1 game against Rek'sai, I would love to pick her up, her kit seems super cool.
I don't want people reading this and thinking "oh he's just whining", maybe I didn't play against her right, so enlighten me. The game I played, we had a lux twitch bot, a Vi jungle, Wu top and I mid as Morgana. I completely destroyed mid lane, got fed and roamed. (at 15 minutes I think I was 10-0-5) Our jungle Vi (who I know irl) had never played against a Rek'say, so we both figured best bet would be to shut her down via camping top lane, which Vi did.. and did well. When "laning" stage was over I think Rek'sai was 0-7-1.
Why I think she needs to be tweaked. (IMO) they need to change the "max amount of tunnels" she can have out, and they need to increase the CD on her ult... here is what we ran into. The Rek'sai we played against built CDR, and ALL she did all game was push top, with 40%cdr she has a teleport (her tunnels) every 66-48seconds. That is SUPER short CD. She'd put a tunnel under her turret, push a wave (or 3 if we are trying to get dragon or push another lane) once we killed her, she'd spawn and "tunnelport" right back into lane..and do it again. And along the way she'd put tunnels up in the jungle, which sometimes we didn't see....and be that much closer to our towers. We did win the game. BUT if we hadn't had the morgana/lux/Vi CC on our team, we would have lost.
So maybe no one is building Rek'sai like this one person we ran into, or maybe we just played against her wrong. But it was super annoying to have to keep such a close eye on a lane that we shutdown so hard.
Here is the game so you can see her build http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1662165375/45342867?tab=overview
It wasn't a "standard" game, because I was also playing with my wife, who's level 17. So normally with that big of a early lead, we should have been able to push harder.. But our twitch was pretty new to the game and didn't understand a lot of the mechanics. Anyways, but like I said, it was super hard to push with that teleport always being up.