I look at the withdrawal reputation first. I pay more attention to how the casino handles complaints than the overall rating. I also prefer doing a small deposit and withdrawal test before putting in more money.

the casino that lets players do bonus buys  during bonus wagering is getting a freeroll , a freeroll for the house.

if the software lets you place the bet, the house should honor the result, if they don't want people placing such bets then they shouldn't be running slots that have bonus buys or they should have a system in place that prevents doing bonus buys. 

iirc when I've played at wildz it's allowed me to bonus buy in this situation too but does show a warning onsite after doing so, do you recall seeing one?

Nope they did not change the rule but made it more specific. Everybody knows you are not allowed to buy bonuses while on an active bonus. You just found a loophole, took advantage of it and tried to walk away after abusing terms. They returned €200 as the system allowed you to do the bonus buy.