I can certainly see an alternate timeline where Alternity became the foundation of 3rd edition. It reads a lot like a systematized version of 2nd edition. Imagine - thief skills and proficiency checks and saving throws and attack rolls, but they all use the same dice mechanic . . . roll-under on a d20, with penalties represented by plusses and bonuses represented as minuses. Then complicate that just a little bit by getting rid of static modifiers and making your plusses (penalties) and minuses (bonuses) into extra dice you roll in a step-system. You roll your Skill +1 and that means you have to roll under your Ability score + skill ranks using a 1d20 + 1d4. The same roll at Skill -1 would compare the target number to 1d20 - 1d4. Bigger modifiers change the size of the second die: +/-5 is 1d20 +/-1d20.
It's interesting, and it looks like it would work, but a d20-system conversion would be easy as hell and make the game much, much better. Because this has all the swinginess of d20 (seeing as how they both use d20s as their primary die), but it adds a bunch of baroque embellishments that mostly just seem to slow things down. You're going to be rolling extra dice and doing extra math and for, like, 90% of use-cases, d20+modifier vs target number is going to work just as well.
But that's just me being churlish. I'm from this game's future, and I'm like, "hey, this is just a less sophisticated version of the thing you guys are going to do next, so I don't see why you're bothering," as if that foreknowledge (i.e. "hindsight") is at all useful. It's a new core system. It's halfway between second and third edition D&D, but it's coherent. I'm sure it has its flaws, but nothing that really leapt out at me, except that routine rolls might be a little too complicated. I could see myself playing this.
Now, onto the real question - how well does it carve out a niche for itself as a sci-fi game? This is tricky for me, because I fully understand that it's trying to be a generic science fiction game, capable of running stories in multiple genres and settings, but I don't think it's really earned the right to do that. Which is maybe a bit of a hot take, and I'll admit, I might be influenced by the game's D&D provenance to be harder on it than it deserves, but I kind of felt like Alternity was taking a very "Dungeons and Dragons" approach to being a science fiction game.
Which is to say, a typical D&D book is notionally "setting agnostic," where maybe Greyhawk or The Forgotten Realms are used as the default setting in some examples, but it is both expected and encouraged that the DM (and lately, players) will develop a homebrew campaign. So it's a "generic fantasy" toolkit and nevermind that it has a bunch of highly specific assumptions baked right into the race and class descriptions about what "generic fantasy" is supposed to look like. Most people (even, to some degree, myself) overlook this contradiction out of an (in my case grudging) acknowledgement that D&D is the inheritor to a decades-long tradition that has fundamentally shaped what modern fantasy looks like. Elves and dwarves and whatnot, all very "generic," don't you know?
Alternity feels like it's trying to catch that same vibe, but it does not have that same weight of tradition behind it. It's out here making very specific choices about sci-fi concepts like cyberspace (it's called "the Grid" and your gridpilot will use their gridcaster to create a shadow to navigate the gridscape . . . it's less "decker problem" than it appears, but not by much) and psionics (it's optional, but if you include it, it's going to work in a very specific way, within a very specific power level) and FTL (every jump through drivespace, regardless of length, takes precisely 11^2 hours, which is an idea I would like a lot more if it didn't imply that an arbitrary unit of time - approximately 1/24th of the rotational period of an unremarkable lump of rock - was an important cosmic constant, but that's probably more of a lingering social contract from my mathematics training - you only leave a value expressed as an exponent if it's exact, if it were just an estimate, you'd say "121 hours"). And so, maybe you could use this book as a starting point for any number of sci-fi settings, but if you're not playing the implied setting, you're going to have to tinker with a lot of stuff.
But like I said, Alternity hasn't really built up the trust (or, at least, the unstoppable cultural inertia) to ask that from me. I actually like the implied setting just fine, and would be happy to play in it, but first I'll need a real setting guide. I'm not going to guess. And I'm certainly not going to make it up from scratch.
Ukss Contribution: Irony alert! Not really picking something goofy per se, but I am picking something based not on how cool I think it is intrinsically, but rather because a quirk of how it was described made me smile. "Jet Ski: A form of recreation on most worlds . . ."
Usually, when a notionally generic system starts in with the "on most worlds" bit, it gets my dander up, but I this time, I was tickled at the thought that jet ski riding was a universal constant, throughout space and time and alternate dimensions, even among alien species. Maybe it's a lingering frustration that Moebius didn't get to ride one on Loki. Maybe I'm more of a sucker for branding than I thought. But this time, I am picking up the gauntlet Alternity is throwing down - I can't speak for "most worlds," but you can add Ukss to the list.