It depends on the RTP of the platform but if you have a number of spins bonuses when deposit I guess there's a higher chance to get a profit like with the bonuses when you hit a good jackpot this is just my personal preference because I do not totally fan with the slot games and base on my experience RTP is just a small percentage but imagine having 30+ bonus spin that might get profitable if you have a good spins.
RTPs are never a small percentage lol. RTPs in slots define in what percentage does the slot would RETURN to its players in each and every game. Hence, the lower the RTP, the lesser players it can attract. Regardless of the bonuses, the RTP remains the base factor on how much they can return. Though it increases your chance since it is a BONUS, it still isn't far different from just having a higher RTP than higher bonus rates.
Basically, RPTs are way better than promotions. It attracts gamblers who really plays and not those who just wants the bonuses based off on promotions. Henceforth genuine audiences.