For someone who is trying to gro you probably want to make sure that you are using a few effective long-tail keywords. This is because the more general keywords will likely be too competitive for you to rank very high in them.

As a general concept, less specific keywords are not the same as more specific keywords. If you have "Uncharted" and "Gameplay" as keywords, that is NOT the same as having "Uncharted gameplay" as one keyword. The former are keywords for Uncharted (on its own) and gameplay (on its own), which are not likely to be all that valuable. (In particular, who would just search for "gameplay"? No one, yet that's what YouTube is "interpreting" your use of that as its own keyword as)

It can't hurt to use something like "Uncharted 4" as a keyword...but it probably won't help that much either. This is because while Uncharted 4 is a very highly searched keyword, it is also a very competitive keyword and it's unlikely your video will rank on the first page. (In fact, TubeBuddy's Tag Explorer ranks it as a 44/100 "bad" keyword due to it being so competitive.)

So, what happens if we add multiplayer? First of all, keep in mind that having "Uncharted 4" as one keyword and then "multiplayer" as another won't work. (Again, those are two keywords, and who would just search "multiplayer") So, if you wanted to use both, you would need "Uncharted 4" and then "Uncharted 4 multiplayer".

But "Uncharted 4 multiplayer" turns out to be even worse. Even though it is a more specific phrase (which is usually a good thing), what happens is that the search volume of the keyword decreases faster than the competitiveness of it. So now you're looking at a highly competitive keyword (everyone uses that keyword) that isn't searched as much. Tubebuddy ranks this as a "bad" keyword 36/100 -- even lower than the "uncharted 4" keyword.

One thing you should get in the practice of doing is going to youtube search and beginning to type search terms and seeing what comes up in autocomplete...this will usually lead you to good long-tail keywords. In this case, Youtube (and TubeBuddy) will both suggest "Uncharted 4 multiplayer gameplay" if you started with "Uncharted 4 multiplayer". This is an even more specific keyword (and still describes your video, which is important), but it only improves the keyword to "average' (48/100). Which isn't so great.

But maybe that's because people don't use the word "multiplayer" when searching? As you put in your own message, "uncharted 4 MP". How about that? Boom. This is 77/100 "good" because it's still relatively well searched, but most people don't use MP in their tag.

And then "uncharted 4 MP gameplay"...boom again. This is 86/100 "very good" because it's searched almost as frequently as uncharted 4 MP, but is far less competitive.

You can then go back and do some other variants. For example "uncharted 4" is a really busy, competitive search keyword. But "Uncharted 4: A Thief's End" is a good (79/100) keyword. By including the full name you can get well searched keywords that have a higher likelihood of ranking you on the first page (because most people don't think of putting the full name in their keyword or description).

Then, another thing I would do is if you have a consistent brand, I would include that as a keyword. This is so when people search for you, you can ensure that your videos appear together (instead of just your channel appearing)...also, this will help Youtube associate your views with each other in the "up next" and "recommended for you" sections. (Otherwise, it might just recommend other people's videos on the same keywords, rather than your other videos whether they are on the same keywords or other keywords).

Keywords are just 1 of 3 (really, it's probably more like 4) basic tools Youtube initially crawls to get a sense of what a video is about. The other two things are the title and the description. (The fourth would be closed captions/subtitles, if you're speaking in your video...By adding your own captions, you can rank for terms that are nowhere in the description, title, or tags, but which were captured in the captions.)

description is meant to be more like paragraphs and sentences. Most SEO blog posts I've read say that longer descriptions (300+ words) are more effective, but most people (as you yourself have noted) don't really use them for all that much. The idea here is that the title and description support the tags (without you doing something incredibly silly like pasting the tags in a list in the description. Don't do that.) So for example, you might say, "Check out the Uncharted 4 multiplayer gameplay recently revealed at [whatever event]" (along with other stuff of course, but see how that incorporates those keywords without just being a list?)

I think the people who do so to include links are doing it more for branding purposes (e.g., to promote things across other social media venues, which is also a good idea to do.)
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https://www.youtube.com/watch?v=DSBpFjCYOl0