Welcome to the main downside of tanistry.
On one hand, it combines the advantages of elected and seniority successions (also, it makes tribal government less horrible). On the other hand, better start praying to RNGsus because from now on kingship is decided by the dice roll!
Seniority has advantages? Sure, it stops gavelkind realm splitting, but I don't see it having any significant benefits compared to the other succession types.
Despite hundreds of hours in the game, I've never actually used Tanistry or Elective (closest that I've gotten was Merchant Republic and Tribal, I've definitely played Venice more than any other Catholic title). If not-your-preferred heir gets picked, which character do you play?
With Tansitry you get to play as whoever got the most votes. With Elective Monarchy it depends whether the person who got the votes is of your dynasty. If they are, then you play as that person. If the chosen heir is of a different dynasty, then you lose the elective title but you lower titles default to Primogeniture.
The great thing about tanistry is that it works a lot like elective, but only members of your dynasty are actually eligible to be elected. No need to spend time rigging the elections to ensure you won't be booted off into a random county somewhere. Tanistry even gives a small vassal opinion modifier like elective.
The problem is that with tanistry everyone gets a vote regardless of the size or rank of your realm, which means that after a while it becomes impossible to meaningfully influence the outcome of the election.
It's pretty much the ideal early game succession though, and the fact that it can be picked even by tribal celts is the icing on the cake.
Slight correction, vassals of two ranks below the liege get to vote. So for a duke that's everybody, for a king that's counts and dukes, and for empires it's dukes and kings.
I personally prefer to use Elective Monarchy and simply rig my realm so only members of my dynasty can vote. Then you have the benefits of Tansitry, but your Strong/Genius son that you've personally groomed to have all the best traits actually has a chance to inherit.