An oil bath will wash debris out of the engine as soon as the debris is generated. This produces the smoothest possible surfaces inside the engine. If you're going to spend time and effort to build a break-in device for your engines, you might as well make it an oil bath.

Personally, I just run my engines super-rich for the first tank, then dump the oil out of the exhaust onto a paper towel so I can see the debris that was scraped off the moving parts. Then I open the engine and use cotton swabs to clean out any debris that might've gotten wedged into crevices, such as the space between the sleeve and the cylinder-head. Here's what came out of my most recent engine:



Before I ever ran the engine, I completely disassembled it, used a diamond file to de-burr the edges of the ports, washed all the components, and re-oiled the components before reassembling the engine. And there were still chrome flakes that escaped my careful attention! An oil-bath break-in would've washed them away immediately.