I have not followed much the dev diaries, streams, etc. related to CK3
For this fact alone I probably wouldn't recommend it for you to preorder at least until you do more research. If you haven't been excited enough to follow the development cycle, which is understandable considering the disappointment many experienced with IR, it would be foolish to preorder something you haven't looked into for a while. You can always wait for reviews since the release is less than 3 weeks away.
lack of content and interesting mechanics
Almost every system and mechanic from CK2 has either been expanded upon, tweaked or gotten rid of entirely. I honestly struggle to think of something that is truly untouched from CK2 that is still in CK3.
Plots has been overhauled into schemes, secrets, and hooks
Characters models are now 3D instead of 2D
Lifestyles have both been expanded upon
War has changed considerably with the change to levies, men at arms, knights, and advantage
Religion and the new faiths system is way more dynamic than in CK2
Vassal contracts is a new tool to manage your realm and replaces pieces of what Crown Authority and Realm Laws used to do
Technology has been replaced with the Cultural Innovations system
If any of these listed items interest you take a look at that specific
dev diary
Navies to transport troops is gone entirely and armies now can automatically embark
No special Byzantine mechanics
No Nomads or Republics either
Less starting dates (only 867 and 1066)
I'm sure I'm missing other things added/changed/removed but this is a good starting point.
How does the AI performance look like?
Not really sure yet. We've only had 2 streams of the game so far. And judging the AI is hard to do without getting our own hands on the game.