With regards to any banned games, it would seem preferable if this is made completely clear upfront when taking a bonus – not buried in the T+Cs.
It would be even better if the casino software had the ability to disable these games when playing under a bonus, or pop up a notice that the bonus would be lost.
Genuine new players are just going to feel cheated if denied a cashout because they tried the blackjack – especially if they aren’t trying to double up a few times.
Plenty of ‘regular’ people just don’t have the time or patience to read the small print.
And as mentioned, the real bonus abusers are already aware of the restricted games.