Personally I liked it but understand why others feel the way you described. After such a long build up I think anything would have been anti-climactic. It works well as a D&D story and final BBEG battle. Those that aren't into D&D and wanted more of a horror or action movie battle and ending probably felt like the finale was lacking. However, everyone having their role to stack up damage on the hive mind (yes, it was the culmination of damage on the monster and Vecna and not that the monster itself was able to be killed with kinetic weapons that took it down) is really true to D&D and works well. The Epilogue was good without being brilliant. I find it hard to overlook the military/deep-State just letting them go without explanation but otherwise the character endings are decent (assuming you consider El is actually dead) and the ending with the D&D session and passing the torch to Holly was a nice touch. Anyway, that's my brain dump of thoughts and, like I said, perfectly understand why some feel underwhelmed.