“Plenty of” is still only a small fraction of the ones they've presented and sold, and while their ship pipeline is the only thing that actually seems to produce anything at the moment, it's definitely not keeping up with the patching pace (which is already rather slow). With so many variants, a lot of it is just the same thing, warmed over.
No. It's a perfect goal for a game. As a descriptor, it's useless. Anything can be fun, but what does that tell us about the thing itself? Nothing, because it doesn't correlate with anything that actually makes the game what it is.
My favourite example is Mercenaries 2. It was a really fun game, especially in co-op. It was fun because it was broken, stupid, buggy, hugely inconsistent, and didn't do co-op well. It was bad in every way, and hilarity abounded. Brown Military Shooter 2017 [pick your favourite], on the other hand, is a good game, with tight controls, balanced gameplay, intensely studied and refined reward mechanisms, and a small army of tech guys to keep the networking running. Chances are that, on average, it'll be terminally dull.
If you want to describe what an actual game offers, you need to describe the game itself. “Fun” is your reaction to what it offers… maybe. It is just as often your reaction to some kind of meta-game (which is the case with Mercs 2 — hence why I like the example), which isn't really about the game at all. For instance, your standard beer-and-pretzel game isn't fun because of the game, but because you're socialising with snacks and drinks — the game is just a vehicle to get that socialising going. To start poking at the game, you could use something like the (now-)classic MDA framework and start digging into what aesthetics it offers — it explains the “fun” you're experiencing in more useful terms.
Also, no, “fun” is not the point of “a game” — just look at Brenda Romero's “Train” — the point of a game tends to be to evoke some kind of emotional state (except maybe boredom), commonly through some form of escapism, and the question is whether it succeeds and how it goes about doing that. But not even that is really assured.
So basically, you have two game mechanics (two forms of shooting) to create one actual dynamic (combat), and the aesthetics are sensation and fellowship. This tells us what the game offers, and if that aligns with what you want, you find that “fun”.
