I think it's generally disappointing that he has one default spawn ending and another one that is locked away behind a dialogue-choice so random, that probably nobody gets it as a natural conclusion to their story (unless you are playing as God-Gale currently maybe). I like the conversations you can have with Astarion about the other spawn and would have loved it if they had been given a little more depth relevance for Astarion's epilogue decision. If the Underdark ending was the result of your ingame choices instead of a weird save-scumming experiment, the poll perhaps would look different too. Similar how there are people who strongly prefer one of Shadowheart's Selunite endings over the other.
Apart from that, it's also nice to see that Astarion surprises himself by doing a decent job at something he didn't imagine he would be good at - and of course that he gets to burn that stupid palace down.
