There used to be 50/50 odd on roulet when the betting sites started few years ago but soon they added the 00 to the table therefore you won't necessarily get your bet back. Also I am sure you will have to very all the original money and only withdraw the winning.
Please don’t take this in the wrong way but in my opinion the end game is to get people to carry on betting afterwards and this is when you get caught in the trap. And the worst thing that can happen is that you win in your first £50 as then there is this perception that you will carry on winning when the reality is that in the long run there is only one winner. I only say this as I got caught in this trap and it was so hard to get out. By the way, this is no reflection on the deal as it is good. I was just sharing something personal. Not trying to offend or annoy anyone.
I am still awaiting £50 from TCB for a CASINO.COM deal back in August 2017. That's 14 months and counting. You'll have to make your own minds up, but I certainly wouldn't trust them with any large purchases again (and yes, I have complained 3 or 4 times).
Normally the t&c's say you cannot place bets that cover the majority of the board on Roulette for example. But every one I've ever done has had no restrictions for bets where the odds are roughly 50/50. If you place 25 x £2 bets of Roulette just betting on odd/even or red/black you will almost certainly end up with between £40 and £60 left over from your original £50 stake. The odds are marginally less than 50/50 so your win or loss should be minimal. I'd say it's worth the gamble to potentially get £100 cashback. Some online casinos won't pay out (the same as other retailers) but when I used betfair they paid the cashback with no hassle
On a positive note, I did one of these Betfair TCB offers a while back, I think it was £60 back if you put on £20... they paid out quickly and I think I ended up about £25-30 up once i took off my original stake. Haven’t played since though, I took the money and ran!