Elective you can lose your titles quite easily. Use tanistry, if I could i'd use it for every game. You're likely to not get the heir you want picked, but you get your pick of the litter if you don't like the current heir. Most people -will- vote for your choice as long as it isn't your child. Usually it goes uncle to nephew then from that nephew to his cousin. (The uncle's son.) Its a fantastic crown law, simply because you get the benefits of elective without the draw backs of losing your titles.
A big problem can be if the voters decide to choose for instance a female in a normal marriage with heirs of a different dynasty, though that may only happen when your dynasty is on the brink as im sure? the voters prefer males
As long as you educate your heirs personally so that you have control over what traits he gets and don't act tyrannical, Elective seems pretty easy to manage, in fact many of my chosen heirs get unanimous votes. It only seems to be problematic if you try to have two or more kingdoms, even then it's usually manageable with bribes and perhaps some assassinations, just harder, which makes me think that candidates born inside the kingdom probably have an advantage over those born outside. Tanistry, on the other hand, is far more unpredictable, while only dynasty members are eligible they seem to favor older people from different branches of the dynasty, and if they pick a different culture candidate then you can lose all your cultural buildings which are very expensive and I'd honestly prefer getting knocked down to a duke or count due to losing an election over losing the money spent on cultural buildings.
I much prefer elective. It keeps the vassals docile and the candidate pool small (and mostly restricted to my own children and a handful of powerless dukes). At that point, I can groom whatever child I want to be my heir instead of hoping that some relative was kind enough to give their genius son a 4-star education.