I think they’re fine. All a web seed is, is a direct server download to your torrent client as if you clicked “download” in your browser. It causes no damage to the torrent if they go down, although if web seeds are holding illegal content they pose much greater risks to those hosting them.
They’re usually used to make the initial seeding go faster, since you can web seed to the crucial first 10 seeders very quickly via instead of piecemeal uploading the torrent to the entire peer cloud parts at a time and potentially never getting the complete torrent out for days and days.