From personal past experience, a vibrating collective is a strong and unmistakeable cue to advise a pilot that he needs to back off a little.
If the collective can be given more resistance as limits are reached (like I said, as on the Gazelle) that also would be fine in my book.
What I don't personally like is a plethora of audio cues (bings and bongs etc) which can be mis-identified in a stressful situation, especially for pilots current on more than one type. Even worse are the ones that only indicate that you have just bust a limit, rather than approached it.