I guess there's two reasons for that, either:
a) Nobody in their right mind would buy one of these projectors, so there aren't many out there, or
b) They work brilliantly, and everyone who does own one is busy watching movies on it rather than asking questions about it on a support forum.
I bought my first projector (a Panasonic PT-AE2000) about 18 months ago, just to see if I ever actually bothered to use it once the novelty had worn off. The answer was 'yes', but side by side with my other recent display upgrade (an LG CX OLED), it really couldn't compare.
Since we're all sat around indoors watching movies anyway, if there was ever a time for a costly upgrade, now is it. Big shout out to my local dealer, PJ Hi-fi in Guildford, who lent me a demo 590ES over the holiday period. Ideally I'd also have demo'd a JVC or Epson side by side with it, but right now that's simply not possible. (If you want an N5 any time soon, join the queue or forget it.)
My own 590ES arrived last week, unexpectedly calibrated by Sony's distributor at no cost to me
(Incidentally - the demo unit was white and looked great... my own unit is black, but it's quite an unusual matte finish that marks very easily, and I do half wish I'd ordered a white one instead. That said, white ones are also on restricted availability right now and I didn't want to be without a projector while we're all stuck indoors).
With the right source material, it's spectacular - but it does seem as though there are quite a few ways to ruin the picture. I'm not keen on the 'reality creation' detail enhancement feature (way too sharp), and the noise reduction filter can do a fine job of smearing away perfectly clear detail across a surprisingly large area.
Turn those off, though, and it's a very revealing device indeed. Like a good sound system, quality source material can really shine, but bad sources are ruthlessly shown up. The BBC's "Seven Worlds, One Planet" UHD looks stunning. Last night's movie (Jurassic Park III UHD) was dim, gloomy and about as far from a "wow" presentation as I think I've ever seen. I've since seen some bad reviews for that particular UHD transfer, but the fact that I'm even bothering to look up BD reviews speaks volumes. For the last 9 years I've been sticking Blu-ray discs into my Oppo BDP-95, watching them on my Panasonic plasma, and thoroughly enjoying them without a second thought.
I've ordered the Spears & Munsil UHD test disc, and I'll be spending some time setting up at least one of the presets. With so many controls available on the Sony and on my DP-UB9000, I don't believe it's really possible to optimise the image by eye just by watching movie clips and trying to figure out what each control is actually doing and how they interact.