I've used Isis cranks/BBs for years and tens of thousands of miles and rarely have issues. In fact I have the same Nashbar Isis BB on at least one of my bikes (and a few new spares), and they have been a good BB (great BB considering the low cost). Isis cranks began their decline in popularity ten years ago, and disappeared entirely once Shimano/Truvativ/SRAM adopted the large spindle/external bearing GPX-style BB standard which redefined cranks and BBs for most bikes.

Isis was developed to remedy an issue with square taper cranks/BBs - their tendency to loosen and become rounded off at the interface. Square taper cranks/BB would give you a more universally-available replacement option, but at greater risk of wallowing out your crank, which would never happen on Isis. Current Shimano/SRAM external bearing cranks/BBs would be a better choice since they are widely available and less likely to follow the square taper failure mode.

I suspect you just got a bad Nashbar Isis BB, although 5,000 miles is pretty good longevity for offroad/MTB use. Keep using them if you're otherwise content with them.