"better investment" is a bit tricky to assess. RCs operate nearly maintenane-free; they last almost indefinitely; and, they can take a lot of abuse abuse. A flail is more delicate; not maintenance-free; they can't really take abuse (not in the way an RC can)- I jammed an 8" diameter chunk of wood in my flail and, thankfully, didn't hurt it (but it took me a lot of work to get it dislodged- had to resort to drilling to break it up): I could hit stuff like this with my RC repeatedly (though not looking to do so) with little concern (other than my nerves being rattled by the racket).
Flails can work well for mowing lots of grass (my requirements) as well as some light brush (also my requirements). If I had to go out to do battle and couldn't afford a break-down (if I were working for money with the equipment I have) I'd be running my RC: time will eat into profits, perhaps less so than equipment repairs would. I have hundreds and hundreds of hours running an RC. Only have about 20 hrs running a flail, but I can state that I am already quite aware of the differences in operation/care.