Oofta
Legend
Sounds like you went easy on them. I buffed Strahd to CR 17 and wiped out a group of 6 level 10 PCS with him. He is an absolute beast in his castle, if you want him to be.
A big factor to the calculation is the DM. Some DMs are just better at tactics than others. Back when I played more public/living campaign stuff, I got to know which DMs I preferred because it would be more of a challenge even though they were all running the same mod.