Lack of CRT flicker is the biggest problem. Demo text scrolls are far more readable when you don't have sample-and-hold blur. There are gaming monitors with strobed backlights but I don't know of any that allow you to enable the strobing at 70Hz.

A workaround for modern hardware is frame doubling then blanking every other frame (software black frame insertion). It halves the brightness, but modern monitors generally support higher brightness than CRTs so this just brings it back to the authentic level. It's possible with FFMPEG filters, although I don't remember how I accomplished it when I tried it years ago.

The importance of CRT flicker is one reason I greatly prefer DOS demos to C64 and Amiga demos. Those were mostly designed for 50Hz PAL CRTs, so the flicker is far more annoying than 70Hz VGA.

I mean even these browser ports aren't at the native framerate. The native framerate for these demos was 70hz as they were all designed for VGA video modes.