Yes, and the rights to a live-action adaptation have already been sold for millions.
Beyond that, fair use doctrine is very dubious, and even if you are legally in the clear and properly avoid the right trademarks and label it as a transitive parody or whatever you are making, you can still be screwed over because youtube heavily sides with the big guys on these things. Their dispute process is a joke, and by even opting to argue your point you risk permanently losing the ability to monetize any of your work. Youtube has basically made an (admittedly sound) business decision that they have way more to fear from big media than some guy with a cellphone cam, so they take the default position that will piss off big media the least.
This is actually something that pisses me off greatly as it hinders a lot of creative works, and the terms on this stuff are stupidly long, but the law is what it is, and just ignoring it is asking for trouble.
Making a parody or live action of assassins creed is perfectly fine as long as it isn't involving the games story line. Do you realize how many people do machinima's and live action versions of games they like?
Wikipedia actually has a pretty good section on the legal aspects of this: http://en.wikipedia.org/wiki/Machinima#Legal_issues
The TLDR is that while game publishers have largely ignored fan made works around their art as they tend to be a good thing anyway, if they do decide to enforce their rights, they would probably win. You need only look at the whole Nintendo/Lets Play debacle to see what happens when a company decides to enforce it's rights over their IP in fan generated content. The brony community is also a pretty good place to look for stories of when IP gets ugly.
Maybe you can make an assassins creed live action and maybe it'll be left alone. Just saying that if for whatever reason Ubisoft decides they want to be dicks, they've got the law on their side, and even if they didn't, youtube will mostly side with the publisher.
There really is no point in worrying about it though until it happens.
When it's your own time and money, then sure, I fully agree. If your taking other peoples money to produce something though I feel ones responsibility changes a fair bit. At the bare minimum I feel the risk needs to be disclosed.