What are the main drawbacks if we choose to use Node-Affinity (sticky MAC) with a load-balanced IIS solution?
We are thinking about using in-memory Session State, so need to evaluate the options.
You may get uneven balancing due to proxies, heavy users sharing the same IP, etc.
And obviously if a machine fails you lose all your sessions when users are shifted to the other machine. A disadvantage that you don't have with a hybrid system with cached state loaded from a shared backend.
Primarily session state information that can get screwed up when balancing between two systems would be the primary thing to look out for. You may want to check on the serverfault meta board to ask about how this website handles sessions and load balancing (and notes for the podcast wiki), as Jeff and Joel have been very transparent about creating, configuring and modifying the ServerFault website and the issues they've run into. I'm pretty sure that Jeff discussed session issues and balance issues before.