Do not, under any circumstances, agree to play this convention. The idea is that you use the bid above 4 of the trump suit to ask for key-cards. However, if this bid is a suit, it's often far too difficult to distinguish it from a natural bid in that suit. You could have detailed agreements to try and ensure you always knew which was which, but no matter how good the agreements are there always seems to be more potential for confusion. Besides, I don't rate key-card asks as being particularly fundamental to slam-bidding, certainly not enough to warrant having all this extra space given to them.
Rusinow (leading 2nd from honour sequences)
Most of the time this is OK; however there are some situations in which you will want to lead unsupported honours, and then Rusinow doesn't work so well. You could agree to make exceptions, for example in partner's suit. But defining these exceptions carefully enough is not so easy, and the supposed advantages of Rusinow over standard methods seem to me to be far too tenuous to warrant all the effort.
Raptor 1NT overcall (showing a 4-card major and a longer minor)
I actually play this at the moment with Mark, but I wouldn't want to play it with a new partner. The problem is obvious - what do you do with a hand suitable for a natural 1NT overcall? Well, either pass or double, presumably, but I really dislike both of these options.
More generally, I really hate playing methods which give me difficulties on hands which "standard" systems would have no problem with. It's incredibly frustrating to be playing some clever gadget, and then have to admit that if you were playing standard methods you would have had no problem at all, whereas your gadget has made the hand unbiddable.
So my policy is to only play gadgets if they come at little or no cost. It doesn't matter if they have compensating advantages - if a convention puts me at a clear disadvantage compared to standard methods on a common hand-type, then I don't like it. That's why I won't play Raptor.
3NT opening showing a 4-minor pre-empt
The more traditional "Gambling 3NT" is OK, but when the minor suit need not be solid it doesn't work so well. There is more variation in the hand types, and so continuations are more difficult. Partner might not even be able to tell which suit is held. The convention would probably work well enough if you had enough time to discuss all the follow-ups and pass/correct bids. But for such a low-frequency opening you probably don't want to put that amount of work in.
For similar reasons I dislike any high-level multi opening. Even playing the multi-2D, which I've been using regularly for a long time and comes up a lot, I worry about all the unclear sequences that it generates.
OK, maybe this one's too obvious. We all know Gerber is a bad convention. But I agreed to play it with my partner in the local league because he really wanted to, and I wanted to keep my partner happy. What I learnt was:
Never agree to play Gerber,
even if it's just to keep partner happy.