I'll throw my two cents in seeing as Undercutters seem to be my thing, I can see the draw of wanting to mount the blade directly to the output shaft but for Beetles i would strongly advise against it. The current crop of Beetleweight spinners are truly scary machines, Lynx's single tooth bar has made it through 15mm hdpe and 3mm (Jamie?) steel, to support that it needed a 12mm high tensile shaft and needed the be run via a pulley. Obviously i can only advise, do what you feel like doing but the power to weight ratio of these machines means that your brushless' output shaft will likely Die a very quick death.
Edit: I seem to have confused Beetles with Feathers
I think my point still stands, doubly so if you're planning a Feather
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