I can relate a story:
I had a hard time using the Delite eyepieces at first.
I liked using them with the eyecups in the down position but I couldn't maintain the exit pupil. I would not only drift in too close to the eyepiece and get blackouts, but also had difficulty holding the exit pupil laterally.
It seems the size of the eyepiece did not lend itself to stabilizing my head position by having the rubber eyecup touch the side of my nose.
It was too far from my face for my nose to touch.
So I raised the eyecup one line, and that helped a bit. 2 lines and the eyepiece was getting easier to use. 3 lines and the rubber flipped up and Eureka! I could easily hold my head laterally at the exit pupil
and I had no tendency to drift too close. I had, essentially, modified the eyepiece to work well with my face shape and eye relief preference. The eyecup touches my nose at that height, and I get the additional benefit of a bit more
shielding of peripheral light.
This is the same problem I seem to have with all long eye relief eyepieces that do not have adjustable eyecups. I had a problem with the XWs until I raised the eycups, too.
But, I don't have any issue with lateral placement of my eye on any of the larger diameter eyepieces, like the Ethos, because when I use them I always locate my head with the contact of my nose against the side of the eyepiece.
And the slightly shorter eye relief pulls my head in toward the eyepiece.
And, if there is even a trace of SAEP, where maintaining pupil position is a lot more critically important, if the size of the eyepiece doesn't perfectly match your head, it can be a problem.
And different people have different curves on their corneas and different depths to the pupil behind the cornea. These can play havoc with eye placement if the exit pupil is small or the eye relief is long. See Clive's comment in post #13.
I found that the effective eye relief of the Pentax XWs was, on average, 3mm less than the TeleVue Delos. It doesn't really surprise me that a lot more people have issues with the Delos in terms of eye placement.
I recall the TeleVue Radians had very long eye reliefs as well, so they were more temperamental in eye placement unless the pupil guide was used (that lateral placement issue again). The eyecup of the Delites
has that pupil guide essentially built into the eyecups so, not surprisingly, people make very few comments about eye placement issues with those eyepieces.
And exit pupil size can also determine sensitivity to SAEP in an eyepiece. In an f/5 scope, a 5mm eyepiece yields a 1mm exit pupil. In an f/10 scope, it yields a 0.5mm exit pupil. If the eyepiece has a trace of SAEP, it will be more noticed in the f/5 scope than it will in the f/10 scope. Many of the widefield designs in the market in the '80s had serious amounts of SAEP, yet they didn't get bad reps because most of the scopes were f/10 SCTs and f/15 Maks. And most refractors were doublets and f/9-f/12 was common. So the exit pupils were tiny.
All of this points out that personal ergonomics will influence one's opinion of an eyepiece. I try to stay away from exit pupil discussions and ergonomic issues like the size of the top of an eyepiece because they
interact so differently from one observer to another. Then there are personal preferences, and that is another can of worms.