UPDATE From Bob, Summer 2020: After quite a few months of testing, switches compatible with the original Terasic PSU have been found. Everything else in this article is accurate though and you shouldn’t use low-quality switches with any PSU (including the alternatives listed below)…
Several users have reported voltage drops when using 5.5mm barrel plug power switches with the MiSTer. Two Three people have now tested them on a multi-meter and the results are shocking “5.1v normally, 4.2v w/switch” –Lo-Fi
Although the switches may “work”, they are dropping the voltage to dangerous levels. Symptoms include boot failures and the fan running slowly or not spinning up at all. I strongly advise against them, even if they “work”. Under-volting electronics is bad, mmkay…
Update: xwred5 (via RetroRGB Discord) also confirmed a 0.75V drop in their switch. The current must be measured when the switch is connected to the MiSTer to get the correct reading. These switches are thus confirmed to add almost 1Ω of resistance!
The alternative I suggest is to use a 5V/3A Micro-USB PSU (w/inline switch), combined with a micro-USB to barrel plug (5.5mm) adapter. For $5, you get a nice PSU with a switch. A larger PSU is necessary for many USB devices anyways, so it solves that problem as well.
5V/3A Raspberry Pi PSU w/Switch: https://www.ebay.com/itm/5V-3A-Micro-USB-Power-Supply-Charger-Adapter-W-Switch-Cable-For-Raspberry-Pi-3/232668902995
5.5mm to micro-USB Adapter: https://www.ebay.com/itm/DC-5-5×2-1mm-Male-Plug-To-Micro-USB-Female-Connector-Adapter-Charge-Converter/362347031846