I’m not sure what you mean with “safety of using RDP”. Windows’ native RDP can run with virtual display drivers, so it can create virtual displays mirroring the client displays and switch the user’s remote desktop session onto those virtual displays.
Anyway, that NoMachine does not provide a virtual Windows display is not because of technical limitations within NoMachine. It’s a question of licensing terms. You may have noticed that all our server products for Mac and Windows connect users to a remote local display which is a physical one. NoMachine captures what the graphics card on the remote device displays. I.e it functions as any other remote desktop software. The exception is Linux. Our free product on Linux, for example, can connect you to both the physical display or can create a virtual display (in the case of headless machines).
Btw, this question has been dealt with multiple times in our forums. For example, here:
https://forums.nomachine.com/topic/enterprise-desktop-with-dual-monitor#post-35185