I have had a number of starts and restarts as a MR in Britain. The main problem is much more prevelant if you convert (you don't get this problem as much in Venice since you don't convert). it also occurs only if you lose the election. Say Venice is a Dukedom, what happens if you create a new Duke title and lose the election? It used to be the case that the second Duke title gets moved to your heir and since you cant have two dukes without a king, your heir becomes independent, sometimes switching government type.
It also happens if you have a Duke somewhere else, and he becomes heir and you lose the MR. Why it affects tribal MRs is that you may have 100s of relatives married to counts, with titles, etc. So you almost always get people with landed titles being selected as your heir.