I'm using Nuendo 12 (it's Cubase really with added post production & game stuff). I've not had any problems at all and on my system, it runs very smoothly
When you say "Lost all my Loops and Instruments..."
1) Do you mean from the right side browser as in my first screenshot?
2) Or, from the main Media Bay (Hotkey F5 on a PC), second screenshot?
3) "Glitches" sounds like it's possibly your 'Multi Processing, ASIO-Guard, Power Schemes and Buffer sizes/settings etc...', third Screenshot.
A few more details might be helpful...Mac, Windows, what Audio unit etc...? Are you running the current update (you can check in your 'Steinberg Download Assistant'?