I too have a crippling dual CPU workstation hoarding habit. Single thread performance is usually worse than enthusiast consumer desktops, and gaming performance will suffer if the game isn't constrained to a single NUMA domain that also happens to have the GPU being used by that game.

On the other hand, seeing >1TiB RAM in htop always makes my day happier.

Power usage is the main limitation of using these as a home server. They have a high idle power use.

Very bad performance per watt and higher maintenance needs. Bad performance per watt generally means a larger formfactor and more noise as well.

On Dell you'll probably be stuck with the original mobo, and their SFFs don't take standard PSUs

Just performance when compared to current generation hardware. Not significantly worse, but things like DDR4 ram and single thread performance show the signs of aging. Frankly for similar $$$ you can get a new hardware from beelink or equivalent.