One problem that I did run into, noticed and corrected quickly: the ends of the blanks need to be milled "square". I had more of a problem on this with Sierra's and large tubes than others. IF the blank/tube seem to be visually "square", it can still be half a degree off and this can cause a problem - described below.
FOR TURNING BLANKS on the "no-mandrel" method:
Unlike a blank/tube on a mandrel - there is not support except where the drive centers contact the bushings and the bushing fit in the tubes. On my first couple of Sierras, As I tightned the centers up to the bushings, the bushings tightened against the tubes. ALIGNMENT comes into play here that is as big of an issue on mandrels at this point. ANY misalignment on the milled ends will cause the bushing to draw up tight against the misalignment. This will result in an out of round blank. TO some people, it may not be very noticeable but I had two that were approximately 1/75th or 1/100th inch out of round, and that is very noticeable to me, especially after assembly.
I checked and checked to find the problem. The bushings were within .002 or perfect in run out. There was a minute' play with the bushings in the tubes that could be forced with end pressure. Then I discovered that the bushings would align to the ends and if one end was 1/2° or 1/4° out of square, the bushing would draw up to that, producing an out of round turned blank.
FOR FINISHING:
I have not discovered ANY problem with finishing using the "no-mandrel" method. OF course, I take the bushings OFF for finishing; otherwise I would have the same problem as finishing on a mandrel with bushings.