Yup when it comes to the stuff of large scale I understand the changes. I still think Blackwater was done really well. I didn't expect massive scorpions and catapults on dromond ships; it's HBO but it's still television, and you'd need a 150m budget to replicate that battle.
With the House Of The Undying...eh. I guess the fear was that if they did it too blatantly it would "spoil" major events in S3 (and future seasons). Still, I wish the writers would trust the audience with prophesies and theories. Lost and other shows proved audiences can handle that information, and obsessive fans will spend time trying to figure stuff out.