Unity is more of a case of bad coding/innfeficient engine than anything power-related.But I do understand what you mean.
In the end I guess it depends on how willing Ubisoft is to port AC games to the Switch.If this rumor is true, then it shouldnt be long to see which games will make up the compilation, since its very likely this is a TGA announcement.
Edit: For a second there I thought Unity had a PS3 version, so yeah, the games you listed seems more likely.Though I do still think that Unity runs like it does because of optimization.
The reason why Unity runs so bad is pretty simple. It's very CPU intensive due to the huge NPC crowds and all consoles have a pretty weak CPU. That's fixed in Syndicate but obviously doesn't look nearly as impressive. It's pretty rare that a sequel looks worse than the previous game.