Rek'Sai's unique mechanic is weak.

She's also really boring as a result.

I guess it sounds cool on paper, a champ that creates a network of tunnels in the jungle and burrows around the map unseen...

But it doesnt work that way, you basically are creating a tunnel with a limited distance than can be totally destroyed (both entrances) by simply standing on it for 2 seconds. Its not really a network so much as a mini teleport pad that takes you from one predetermined spot to another, and in most cases is made obsolete by the fact that you can just use the E to cover distance in the vast majority of cases. Whether a tunnel you previosly created will suddenly have some tactical value is going to be a matter of chance most of the time.

Setting up tunnels for quick access to contested areas is all but useless, because those areas are contested... Someone on the enemy team is going to see the giant glarring entrance on the ground inside the dragon pit and they'll just stand on your hole for 2 seconds and its gone. Obviously the enemy will know that a RekSai is on the other team and they will sweep these high value areas for tunnels whenever possible...

...This brings me to another issue. The tunnels are all but worthless in a situation where your team has full map control, all those tunnels are just going to be set dressings, you dont need them to get the jump on anyone, and you can just E over a wall if you really need to, no need to find a previously made tunnel.

Its no better if the enemy has full map control. Even if you manage to get a tunnel behind enemy lines, it is likely to just decay, or get reset when you use E for mobility (which you will... all the time) Your tunnels are far from permanant, they can be destroyed, they can decay, and they can be overwritten if you use E too many times. Having one available in the right spot when you are zoned to your base is highly unlikely.

Then there's the ult..., its a teleport that announces itself to everyone and is generallu more relaint on chance than tactics. There will be times that you have a tunnel right where you need it to flank the enemy, but I dont think something so reliant on circumstance is very valuable in a game like this. Basically it isnt very interesting, and often not highly impactful.

Outside of the tunnel mechanics... the champ is pretty generic, none of the other skills offer anything unique and there are better versions of them on other champs less reliant on circumstance.