You can also use Lossless scaling with it, but only the upscale for even more performance (it also fits the game into your fullscreen if you use a lower resolution in game than your monitor when in borderless mode). The frame gen option of Lossless Scaling is very buggyYes.
FSR and DLSS are close in performance but DLSS gives you slightly better quality. Frame-gen is the main advantage, it doubles your FPS, and when it comes to 20XX - you can use FSR frame-gen but not DLSS frame-gen. So, you can use DLSS upscale and FSR frame-gen (they are separated in Skyrim Upscaler).