I love that dock. I had four HDDs in two of these docks connected to one of my servers for a bit. Same experience. A bit slow for large transfers but fast enough for HD streaming. I think the space between the drives allows for enough natural airflow to keep them relatively cool. I think the hottest they got was 63 degrees Celsius during a large transfers. Usually stayed a bit below 60. In my janky homebrew NAS I see temperatures around 50, give or take 5 degrees. The use case should really determine the build so that may be the best option for many, especially with budget constraints.
Unfortunately Dell pulls some shit with their PCs. Their boards are usually almost standard as @catloaf points out. I had a Dell Optiplex 990 that had a board advertised at ATX… but a few of the mount points were a few centimeters off. I thought I might be able to make do without a few mounting screws until I realized the cooler was actually designed to hold the board in place and would not be transferrable. I thought about getting a new cooler but the board was designed so the CPU was too near the edge of the case to fit most standard coolers. Not sure if they do this on purpose to make their parts ecosystem essentially closed or if there is some cost benefit reason but it blows either way.