Update: Ubisoft has pulled the support article cited below ("for the time being") and told Gematsu in a statement that it is looking into the potential that it may contain "inaccuracies involving the Ubisoft titles that will be playable on PS5."
Both Xbox and PlayStation have kept backwards compatibility in mind going into their next big console launches, but a handful of Ubisoft games have highlighted that there might be some hidden complications in that process on PlayStation.
According to a support article for Ubisoft Connect, nine of Ubisoft’s games for the current, Xbox One and PlayStation 4 console generation won’t work on the PlayStation 5. Interestingly, no such restriction exists for Xbox consoles; Ubisoft says its entire catalog of Xbox One titles will work on Xbox Series X and Series S.
“Most of our back catalogue of games will have backward compatibility between the next and current generation of consoles, with a few exceptions," reads Ubisoft’s page on cross-progression and cross-play. “This means that you will be able to play your Xbox One game on Xbox Series X|S, or play your PlayStation 4 to PlayStation 5.”
Assassin’s Creed Syndicate, Assassin’s Cred Chronicles Trilogy Pack, Assassin’s Creed Chronicles India, Assassin’s Creed Chronicles China, Assassin’s Creed Chronicles Russia, Risk, Star Trek Bridge Crew, Werewolves Within, and Space Junkies are all listed as games PlayStation 4 games that won’t work on PlayStation 5.
Star Trek Bridge Crew, Werewolves Within, and Space Junkies all notably never saw a launch on Xbox One (and thus won't be on Xbox Series X|S either), and are all PlayStation VR games (though PSVR does in fact work on PS5).