Exactly this - Hannibal is good enough, but he can't be trusted not to get sent off. I can accept that in a player who is determined, wants to win tackles etc - like a Rooney, a Keane, a Gerrard - but when all those yellows come from retaliation or complaining to the referee - that's a problem. Because a) opponents know that 2-3 tackles on Hannibal and he will get at least a yellow - simply because he will retaliate or complain to the referee. and b) determined players wont lose focus after taking a yellow - it's just their style. Players like Hannibal who clearly are affected when they are tackled - will be very easy to provoke into losing focus and not doing their job.
So if I was Rangnick and wanted to try Hannibal - maybe tell him the following: if you get a yellow for doing stuff like this, we will sub you immediately - until you learn.