Minisforum Neptune Mini PC: Ryzen 7 5800H, 6600M Graphics, 16GB RAM, 512GB SSD

Absolutely. There will always be the "4K 144hz ultra settings or gtfo" PC elitists who will insist that one must spend at least $3000 on a huge RGB-studded water-cooled monster PC with a nuclear-powered graphics card that costs a month's salary by itself in order to enjoy PC gaming at all, and that crowd would certainly scoff at something like this, but for the more level-headed, the machines linked in this deal in particular will be able to play any game on the market just fine, if you're willing to adjust some graphical settings down a bit in the latest titles. Obviously, no gaming PC will cut the mustard forever, but it'll be at least a few years before we see any titles that would be outright unplayable on this thing, certainly at least until the end of this current console generation.

To be very broad and reductive, you can expect the overall performance level of these machines fall somewhere between the PS4 Pro, and the PS5. On top of that, it's small, quiet, and consumes relatively little electricity. This is a killer bargain for someone with moderate expectations and requirements, who isn't fixated on ultra-high settings and framerates in all of the latest titles.

Extremely good actually. There are multiple reviews of this model and slightly higher speced model HX90 with 5900HX + RX 6600M/6650M which you can check in YouTube.

Not really, at least as far as new prebuilt PCs that are ready to go out of the box.

Not really, no. The CPU in the MXZ machine benches around 10% faster, but all other components being equal, the 6600M soundly beats the 1660 Super in every single matchup, sometimes dramatically so (the 6600M and the 6600 are essentially the same part with the same overall performance, so don't worry that this video shows a 6600 instead of a 6600M specifically):
https://www.youtube.com/watch?v=DPN3REiWoNU

The trade-off is that you can't really upgrade the GPU or CPU in these Minisforum machines. But honestly, the upgrade path for that MXZ machine on Newegg is kind of limited as well. You could spend $350 on a 5800X3D to drop in there later on and wring another 10-20% extra performance out of it, but other than that, the AM4 platform is finally reaching EOL, so there's only so much further you can take it. Likewise, you can drop in a faster GPU, but by that point you might be better off to just think about a whole new machine based around AM5 and next-generation GPUs.

The Beelink machines only have the integrated graphics that are built into the Ryzen APU. These Minisforum machines linked here also have a 6600M dedicated GPU. The performance difference in games will be massive, like a full order of magnitude between them. Compare Doom Eternal running on the 6600H alone, like in the Beelink:
https://www.youtube.com/watch?v=0r3pSLxEg78

And here with a 5800H plus the 6600M, like in the Minisforum:
https://www.youtube.com/watch?v=VDiuhq2gInQ

There's no comparison whatsoever.

Correct, but this particular CPU family is nearing the end of its lifespan anyways. AMD is now introducing the AM5 socket, which will be the standard within the next couple of years. This machine will be a solid performer for the next few years, until the AM5 platform stabilizes a bit.