I do not own the new Opticfilm 135 so I can't comment on its performance regarding focus. Also, of course, one cannot exclude sample variation.
I however still own an older Plustek model, the 7500i. I have compared its scans to those of two other scanners I own which do have focus adjustments, the Minolta Scan Dual IV and the Nikon Coolscan 8000.
The results of the Plustek are so close to those of the Minolta and of the Nikon as to be practically indistinguishable. My null hypothesis is nobody would be able to tell the scans apart in a blind test. This suggests to me that the focus plane of the Plustek is perfectly calibrated towards the plane of the negative suspended in the included holder (which is excellent at keeping the film flat).
The only reason I've set aside the Plustek, settling for the Minolta for my 35mm scanning is that the Minolta is motorized and considerably faster in scanning.
Overall, my experience with the Plustek matches with the one this guy had
(he tested an 8100, which is an 8200 without ICE)