I just want to point out that this post pretty much sums it up. I can't tell you the last time I ever used her R for anything other than rushing back to jungle or the occasional gank setup. Yes, the passive attack speed is very useful, but stop and think about what she actually brings to a fight!
Q(above ground) - Primary damage dealing ability, incredible wave clear and AoE damage, requires you to be on top of the enemy to use effectively.
Q(below ground) - Skill shot with mediocre damage on a short cooldown, AP scaling (useless for any real build), gives vision of target for 2 seconds. Currently I think it is bugged and vision isn't instantaneous.
W - Great AoE knockup, relatively short range, not repeatable for a few seconds, requires you to be underneath your target for maximum effect. This right here is why Rek'sai will either destroy you or get crushed. If you do not build tanky, you will not have enough time to get on point and use this ability. Also a good point to mention that the sustain from consuming fury is really good after the first few levels.
E(above ground) - Decent damage, Great damage when full fury, Rek'sai's only other dueling ability, requires charging for maximum effect.
E(below ground) - amazing mobility on a relatively short cooldown, reusable tunnels, easily spotted and destroyed. Actually useful as a form of indirect vision control, and bait.
R - amazing passive attack speed bonus. Ultimate is useful for repositioning but requires pre-placed tunnels which can be a huge negative because most of the time enemies will destroy your tunnels on sight. Easy to hide one end of the tunnel, not the other. Great for rushing back to your own jungle and farming.
The point is: only two of these abilities provide any meaningful damage, with a kit designed for hunting and destroying fairly quickly. The more time Rek'sai has to spend fighting a target, the more likely she is going to die. By taking away from her damage, you make it that much harder for her to do her job effectively.
I want to make it clear that these changes hurt JUNGLE Rek'sai much more than they do lane Rek'sai. Jungle Rek'sai will suffer tremendously.