Nice build for a home brewed speed control box. Downsides I see, if you had to buy the materials first, you could be looking at $15-20 in parts, but if you only need the speed control bit, its about $6 (some are $13). To buy one outright, at least $25.
https://www.homemadetools.net/forum/diy-speed-contoller-box-ac-motor-75313?homelistings#post142227