The most significant changes to the database library were internal or in the form of additions to the API, so most things will probably work. The big issue is that it's an untested combination, and you'll probably find that any issues you do have will be difficult to troubleshoot, hard to track down, and definitely unsupported. You'll also still have most of the security issues in CI2 to worry about.
It's actually pretty amazing how many of the CI3 components can be swapped into CI2 with very few issues, but it requires a lot more attention to detail than just upgrading the whole thing at once.
