What will I lose if I install Cyanogenmod? I plan to install CyanogenMod on my Samsung Galaxy Express 2 (Sm-G3815). I have made a backup using Titanium Backup, by choosing the "Backup all user apps + system data" option. 

I am not clear on what exactly will be done to my phone, what will be deleted and what left alone. Presumably, the backup I made will allow me to restore installed applications. Assuming that the ROM works as expected (and yes, I am aware that it might not) and everything goes well, will I also be able to restore


my contacts?
existing conversations from chat apps like WhatsApp, Telegram and Viber?
phone call logs?
photos?
sms messages?


Will anything still be there when I reboot into the new ROM or will I have to restore everything (except the data on the external card) from backup?

I have quite a few of my apps on the external SD card (moved there by choosing the "Move to SD card" button on the application manager). As far as I can tell, this move means that some of their data is in the external card and some on the internal storage (as reported by the application manager). Will this cause any issues when I try to restore from backup?
The phone memory will be wiped by the Cyanogen install. Everything will be erased.

If you have a separate SD card, that should not be altered, so if you apps store data there, you may still have it after you reinstall each app on CM, or you may be able to manually copy over the old data and thus restore it.

Contacts and call logs are synced to your Google account so unless you disabled that, once you install CM and put in your account info, they should be restored.

WhatsApp, Telegram, and Viber history: Depending on the app, this may be stored in phone memory, the SD card or your account with the app's vendor. I believe for the three you mention, all keep your data tied to your account in their own server, so you should be able to recover it once you install the app on CM and put in your account.

Photos will be saved if they were on the SD card.

SMS's will likely be lost.

When I flash ROMs, I eliminate the ambiguity by just manually backing up whatever is important, and wiping the phone clean. Much simpler, and does not rely on trusting some backup app developer to get everything right.