They did a fabulous job of the Star Trek transfers (I’m currently watching the original series).

However, some studios put very little effort into their film to HD transfers. Last year, I watched Buffy, the Vampire Slayer on Disney+ and its transfer from film is woefully bad. For certain scenes, the picture quality looked like upscaled standard definition and these transitions were very jarring. Also, whatever filter they used for grain removal made the flesh tones and facial features look “wrong”.

Even worse was the wholly unnecessary conversion from 4:3 to 16:9. The resulting composition of many scenes was distractingly bad. At one stage, they the second camera unit can be seen filming the action from the side!

Edit: Fortunately, Disney+ have made The Simpsons (another transfer from film) available in 4:3 – as well as the default 16:9. There’s a setting in the UI to play it in 4:3.

An this unfortunately also makes a remaster of VOY and DS9 very unlikely. I also read somewhere that they used more digital effects and 3D renders which makes a remaster even more difficult, as it would basically require to update or even redo them.