When you buy a "BU" coin from many dealers, including big advertisers in the national magazines, you are lucky if you get a no-problem MS-60. More frequently you will receive an AU-50 to AU-55 "slider" with cleaning or other problems.
Some reputable dealers call a coin BU if it grades MS-60 to MS-62. Choice unc will be MS-63 to MS-64, and gem is MS-65.
I've purchased raw coins from Stacks auction house. A quarter that they graded Choice BU was slabbed at MS-65 at PCGS. A
Morgan dollar called Gem BU also got an MS-65 from PCGS. A $10 Indian gold piece they called BU (no Choice, no Gem) graded AU-58 at ICG.