I can support that statement.
I can play siege if I join an army without major issues (FPS drops are brutal when the games "turns off" the GPU for some reason during siege, but I guess devs know about this already),
but if I am a leader of an army and I want to lead the siege by myself, or I want to send the troops to do the work, the game crashes immediately. There is no workaround - changing settings does not help..
Tested with Razih city and Sahela castle.