I really like the ideas of the new features in the expansion, but they all seem to suffer from lack of testing in development. HRE control and Cultural Tradition are both way too easy. Especially the former should be VERY hard.
Personally, I think there should be ways to get the imperial reforms reversed - for example, it could reverse one step automatically if a new emperor takes office with low authority or low prestige. The Empire losing centralisation under weak emperors seems quite reasonable to me. Historically, the HRE did actually become more decentralised over time rather than more centralised.
All this said, however, let me mention two things that really work well in HTTT:
- The extra AI manpower on Very Hard difficulty. This makes the game a lot tougher.
- The relative lack of super-blobbing, probably due to the new peace and CB system.