Their "app store" is...filled with cryptocurrency junk

I don't consider that a negative. If you don't want it, you don't have to use it. Although being it is a for-profit company there is always the possibility that they start pushing this kind of software.

requiring your own GitHub repo

This isn't really unusual. Yunohost does the same thing. Presumably to ensure compatibility and ease of use.

The hardware is half as good as a used 1L mini-PC that you can get on Amazon for $170

This really misses the point of the product entirely. It's intended to be an "out of the box solution" and the hardware purchase funds development. Same goes for something like HomeAssistant yellow. If you have the capacity to build your own from scratch, this product is not for you. I assume the hardware is also intended to be efficient power consumption like a RPi.

Also the OS is FOSS and can be installed on whatever hardware you want.

The Umbrel has an HDMI port hidden inside, so if you want to use it you must cut a hole in the side for it.

Not ideal but I've never seen anyone use a server with anything but a web interface. They're probably just using off the shelf hardware and putting it in a case.

I asked for a refund and was denied since they don't offer them to anyone. Huge red flag!”

Agreed there.