I bought a bunch of NUCs 18 months ago for some projects. They're no longer in use. A friend asked if I was selling any. I realized I might be able to sell them at cost or a profit. Crazy times.
There are generally lower-cost alternatives for sourcing the third-party products they carry. Their first-party products are usually more expensive, but they're of higher quality than the competition and come with better documentation and guides.
Both. Microcenter tends to sell Raspberry Pi hardware at MSRP, and occasionally on sale a couple bucks below MSRP.
Adafruit tends to be a bit more expensive. It's never bothered me since I like the company and its service, but I think pretty much anything you buy there ought to be purchased with the thought that you're prototyping and if you want to buy many parts for a final product, you'll eventually want to source parts somewhere else. (or, I assume, call Adafruit and try to negotiate a better price)
It's understandable enough why both companies are this way. Microcenter is a retail chain and can do things Adafruit cannot. (in truth, they are partners on some things, so it's more complicated than that - the point is that Microcenter is a much larger company)
I've never minded paying more at Adafruit to help support them. The support they have provided, with documentation, examples, libraries, etc. has been immensely valuable to my hobbies and work.