Denuvo is a very complex anti piracy system for games that is pretty controversial. There's a lot of evidence that it affects performance and it forces games that wouldn't otherwise need Internet to be activated online regularly.

It's the kind of thing that a reviewer would mention and that some people would use in their buying decisions. Sneaking it in after launch is going to make some people pretty mad and I'd feel used as a reviewer.

Denuvo is like having to call your helicopter mom every other minute to make sure you still have the right to play.
If the call fails, or she doesn't pickup, or if you can't call for any reason (maybe your in the woods and have no service) you're instantly teleported into a dark room and all your toys are gone because everyone assumes you're a criminal now.

Denuvo is always online DRM software, that usually results in performance issues (reduced frame rate, increased latency, stuttering, etc.).

In this case it appears Ubisoft ~~avoided~~ tried to skirt the potential bad press from performance issues by delaying the inclusion of denuvo until after people had bought the game/early reviews came out.

I guess they did it since Denuvo is generally known to cause performance issues in games.

So, reviewers gave scores on the denuvo-less game, which would have better performance, thus better scores, then they patched denuvo into it, so that they will get their drm and any performance drops will not play a role in any low scores.

But I can't understand why reviewers can't update their review... maybe it's expensive for major reviewers?

Denuvo bad.

In the (vain) quest to make people stop pirating, it goes so far (admittedly also comes the closest to "working") that it starts causing significant side effects. It's also apparently always online, which is a historical pet peeve for a lot of people: it doesn't add any value to the game, but it does add a buttload of possible extra ways for the game to crash or become unavailable. With no benefit to you, the player, and not much you can do about it, other than playing the games of someone who's not quite as much of a dick.