We used United miles to go ORD-DBV via DUS (flying LH) with DUS-DBV on Croatia Air. It all went smoothly even though we had a very tight connection in DUS. Kudos to LH for our bags making it on a 30min connection there. The Croatian Air check-in desk was staffed by LH personnel by their uniforms.
My experience was that going to DBV was an excellent value for United miles, since the ORD-DBV fares I always saw were around $1800, so my 60000 miles (x4) were put to good use. There was good award availability. I think I finally ticketed in Nov for the following August for 4 tix.

We couldn't avoid it, but I'd recommend not going during August. DBV was really hot this month.

Montenegro is an easy day trip from DBV via rental car ($80) or tour bus ($80/pp).