I've played it twice through now. The AI seems pretty poor and the game is "streamlined" vs MOO2. No spies, no leaders, no race design. But combat, man, it is just like MoO2 and great for it.
Regarding it being "streamlined", I would argue it's the same complexity as MoO2, but it just removes non-choices from the game. By non-choices I mean "Do I get autominers, or the other tech" That's not a choice, you get the autominers. Anyone who's played MoO2 more than once knows most of the tech nodes aren't a choice, there's an obvious best, then everything else. Maybe if you're going beams vs missiles you make different choices, but that's about it. So SiS removes the non-choices to leave you with spending your time doing the 4x thing.
Overall I really like it, it scratches the MoO2 itch, sort of. It's not a buggy trash heap like StarDrive 2 (so much potential wasted!), but it also isn't crazy expansive like Distant Worlds.
So if you liked designing and fighting with ships in MoO2, you'll love SiS.