OK, I made my own research and made some math. Let's say I have 100% bonus with 30X WR on the bonus only. I make a deposit of 100$ and can play with 200$ but I'll have to wager 3 000$.
Then I play slots with an average RTP of 96 %. That means that theorically, when I bet 100$, I get 96$ back on average and that the casino keeps 4$. Now, I have to bet 30X 100$, mathematically speaking, the casino will get a return of 30 X 4$ = 120 $.
That means that for every 100$ of bonus the casino gives you, it gets 120$ back. It's still a negative expectation.
To have even chances, the WR needs to be no more than 25X if you play 96% RTP on average. If you get a 30X bonus, your average RTP needs to be over 97% on average.
My conclusion, mathematically, it is better to play without a bonus unless the WR has a positive expectation vs the average RTP of the slots you play.
Any comment ?