1. It is too long to watch. 20 games are too many. I recommend halving it to 10
   
2. There is not enough time per game to get a proper look at. Splitting it into 4 smaller viewports isn't a solution, more on that later.
   
3. There is no indication of the previous game once the time for that game is over. I recommend keeping the last 2 games's links on the background , possibly out of accidental clicking range. Moving them to the side and making them smaller as they are unloaded for the next games is a possible solution
   
4. Having 4 viewports showing more or less the same footage is useless (See Beatsaber)
   
5. Having 4 viewports showing clips of a cinematic trailer without actual gameplay is useless (See TW:Three Kingdoms)
   
6. It should be made clear that only the top left viewport has audio. Highlighting it should be something to consider, especially since it is important enough to warrant having sound over 3 other clips
   
7. Having highlights with sexual content or nudity. If they're risque enough to warrant a warning on the store page, it's probably not great to have them on highlights without any warning.
   
8. Above also applies to Adult Only games. Separate entry in the list for good measure.
   
9. It is not great as a general list feature or even as a genre restricted showcase, as most highlights would end up in jacks-of-all-trades games by "AAA" publishers, burying the chance of many indie titles at fame.

It is possibly a better tool for publishers to showcase their seasonal/upcoming/best titles in their publisher/developer pages with the help of steam, or as "rewind" kind of deals with some long standing game franchises during deals.
It can be shaped to be a good tool in general, but in my opinion it is not worth the effort as there are better, more direct ways of doing so without having to encode hours of video clips.