Careful with 15x, when the standard is 30-40, some 15x means:

15x bonus + deposit, effectively 30x.

Or, 15x bonus but only on slots or some other high edge game, so no counts on low edge games like Blackjack or roulette. I would rather have 50 x on roulette than 15x on slots. Anyone who played slots knows this is a huge house edge and you'll not even make 10x easily on slots unless you get extremely lucky.

For reference from their FAQ: https://www.playhubcasino.com/promotion/102/double-your-deposit-every-monday-code-monday
1. "Terms:  the deposit + bonus must be risked 15times (x15)!"
So deposit+bonus = 30x bonus