I just found this out because I just did it this weekend. Needed tickets to Mexico and needed stopovers in both directions. BMI fit the bill perfectly because mexico is priced same as domestic US, and its the only airline I know of offering stopovers in both directions. The allowed PVR-IAH-DEN-EWR also so there is at least some leniency on the point of stopover.

Only a few things I can think of for caution:

You can only buy 24K miles per year. Though I think you can circumvent this by establishing a family membership or just redeem one ways and have a spouse, child or buddy also buy miles. Or just redeem every two years.

US and UA domestic F count as F awards and require 250% miles. CO is C and 150%. On my award I had to avoid UA and US or sit in coach.

BMI uses and Indian call center though I thought they were pretty good. They operate on london hours which means they are open 2am-2pm for me.

BMI does not hold award seats. I took a leap of faith buying miles that seats would still be there. Miles posted in about 8 hours at 9pm. I called call center at 2am when they opened and was able to book seats.