To put it simply, 50w can be filled in 40 minutes, and 30w can be filled in an hour and 10 minutes. That's the big difference!
Oneplus 50w wireless charger can use 30w. As long as your mobile phone supports the wireless charging function, it can be used, if the wireless charger protocol is also supported, the charging power is no problem, because the power of its charger can be backward compatible, but the charging speed will be slower, and there is no problem in other aspects.