Can I restore Titanium App Backups from a phone with Android 4.4 to my Android 6 Phone? Im backing up My Galaxy Note 2 with Titanium Backup.
Is it possible to restore Apps + App Data on my Huawei P9 running Android 6?
And maybe even more important: Is that adviseable and will work without problems?
As requested by beeshyams, I'll sum up my comments to an answer here:

There should be no problem as long as you stick to user apps (i.e. apps you've installed yourself – as opposed to apps that came pre-installed with the device). I've done that for years without any issues – though I must admit I've never skipped major Android versions, but always upgraded step-wise (i.e. 2.3 › 4.0 › 4.1/2/3 › 4.4 › 5.x – which means I've skipped 5.0, and maybe some (but not all) of the JellyBeans).

However, it is strongly recommended against restoring system apps and their data across different ROMs, especially across multiple Android versions, as this almost surely will lead to trouble. I've experienced such in the past when just trying to restore data of some system app. So if you need to include system apps and/or their data, first check for alternatives, in this order:


SMS/MMS, APs, contacts, call lists and some more: Use TiBu's XML backups for those. Restore those XML backups, but never directly the app and its data.
apps not covered by XML: If you cannot do without, take extra-extra care. Always be prepared you might need to do a factory-reset. Try avoiding the restore of system apps (which came pre-installed) the best you can: those are more often closely integrated with the resp. ROM than not.
If you really have to restore their data, make it the first thing in your restore process (so the factory-reset doesn't hurt as much), and do it one-by-one each time followed by testing for side-effects. If it breaks something, skip that backup and rather shed some tears1. If you instead want to follow the hints from the footnotes, make the affected app the first to restore data from after your next factory-reset and only go on when you either succeeded including testing for side-effects – or gave up and did your subsequent factory-reset.


Also see beeshyam's answer on How to restore from Titanium Backup with file on computer? for further details.



1Before abandoning it, you can try enabling some troubleshooting-options: In General › Settings, pick the first item in the "Troubleshooting" section (labeled something like "app processing mode") and change that to "AUTO, indirect". That should try to update the tables inside the apps' databases instead of replacing the corresponding SQLite files, thus keeping the table structures intact (if I'm interpreting this correctly that is; in older versions, TiBu had an option called "Migration Mode" at this place, which is gone without a note in the Changelogs – while this item suddenly "popped up", sounding like what "Migration Mode" was supposed to do). You can also check with additional settings from the Troubeshooting sub-menu: especially enforcing system SQLite helped in several cases.