Okay I'll bite, I enjoy playing Rek. You're being very, very selective in your descriptions of the abilities btw. Have you played as her at all?
Point for point:
Burrow
Because burrowing removes the possibility of auto attacking and by extension the Q. Unless it is used immediately again fro the knock-up (which cannot be used on the same target on an internal cd, similar to Udyr Bear stance) it provides speed and health return in exchange for the ability to lifesteal and see. Unless you're going AP Rek, burrowing in the middle of a fight is sacrificing a lot of your damage for a small survivability boost.
True damage
Small correction, it isn't %health, it's 80/90/100/110/120% of her AD + 1% for every 1% of her fury, and true damage only when at full rage. This does several things, it forces a player to wait until full rage for true damage, or be forced to use it early for a still respectable physical damage. Even with CDR this ability's long cooldown makes it very rare to use this twice in a fight because it will almost never be used at the beginning. While there are exceptions like wandering into her after a jungle mob (in which case she has full rage) or standing right next to tunnel (for w/e reason) this will be used only used once in a fight.
Yes the damage is ridiculous at full rage, but she has to get there first.
AoE buff
If the buff (which lasts for three hits) lasts that long, you haven't been using it (auto-attacking), so the point is moot. The only real reason I imagine her buff lasts that long is to allow the last hit or two to travel between jungle camps. In a teamfight you will almost always use it up via the hit-limit and not the time.
Resource
There is resource management, but it revolves solely around her E and not her entire kit. Yes there is some debate about manaless champs staying power over champions that have a battery, and I do think mana champs can be tunes a bit better in the current state of the game but I attribute that more to an overarching problem than Rek'sai by herself and won't be fixed by re-tuning her.
Global ultimate
Yes it sounds like it's OP, but realistically it isn't used as much as you would think. Her ultimate has one prerequisite: tunnels. If you're team is allowing tunnels to be made behind them and they leave them there long enough for her ultimate's dash to be significant then that's more a problem with your vision and awareness than anything. It's very rare and I would argue difficult to leave a tunnel in an opportune area and have it last long enough for me to be far enough away that the ultimate makes a true difference.
My advice: play her. A lot of the abilities sound pretty crazy on paper, but you have to experience it for yourself to see the kit's strengths and weaknesses. You can only glean but so much by going against it.