I'd dispute that last part. At the very least, it depends on the game.
Cyberpunk 2077 with 4x frame gen is wonderful. With all the detail settings cranked to the max, it's a gorgeous game, but at that level, it even brings a 5090 to its knees. In 4K, without MFG, I often only get ~40 fps. With 4x frame gen, I get around 140 fps which is a significant improvement to the experience, and I don't even notice the artifacts unless I'm looking REALLY hard for them.
On the other hand, MS Flight Sim 2024 with 4x frame gen looks great unless I enable labels on landmarks and other aircraft. During camera angle changes, they get very noticeably distorted. The cockpit looks fine, though.