From my viewpoint you either select browser or you select mac/linux/android/ios.
The first to find browser games to play without downloading. The other options to find games for your system, since most games are made for windows and may not have a port for your system. So ability to filter for them makes a lot of sense. Also you actually browse Itch with these systems. You do not browse Itch with a dreamcast.
Filtering for retro consoles and special hardware on that level is awkward. I estimate about 100-200 platforms there would be projects for. So you would have to use some kind of dropdown box anyways. And we already have that with the tag box. And that one has aliases. for example https://itch.io/games/tag-commodore-64 is also reachable under c64 and under commodore64 without the space.
So in my opinion the only "negative" points of doing it with tags is, that there are some games under a tag that actually do not provide a native file for a platform. And the games have one tag they would reserve for tagging the system. But they will use that tag anyway for all the people that will search for those games under tags.