The LS820 is designed to be used with an ultra short throw screen. Ultra short throw screens run from 0.6 to 0.35 gain; making the excessive lumens and lower black levels suddenly right about where they should be. You can read the LS820 owners thread to get an idea of what that actually looks like vs. a review being used on a "normal" screen. The reason for the higher price tag on the LS820 is because it is laser driven; has little to do with be ultra short throw.
A closer comparison to the BenQ HT2150ST would be the PX800HD which is far less expensive than the LS820 and isn't as bright. Not surprisingly it does great in light controlled environments. However it doesn't have quite enough lumens to push past the punishing gain on an ultra short throw screen at larger dimensions so you your whites will look a little on the grey side etc.
The biggest penalty of ultra short throw vs. short throw and normal throw projectors is that ultra short throw is absolutely dependent on having a good quality screen/surface. Any imperfections will warp and skew the displayed image notably due to the high incoming light angle. Normal projectors by comparison are pretty forgiving in this regard. Beyond that, ultra short throw units tend to have weaker light uniformity, but obviously the higher end units and even the relatively cheap PX800HD seem to be bucking this trend.
The 3 big advantages of ultra short throw are:
1) Easy setup - using an ultra short throw wall mount or just plopping it on top of your entertainment center or table is a pretty simple task.
2) People can stand up near the screen - if you are doing more than just movies there is a higher chance of people getting their heads in the line of fire of a normal projector, ultra short throw lets people get up and dance right next to the screen without issue; in business cases it allows white boarding use, in sales it allows interactive displays among other creative uses
3) Vastly superior ambient light rejection options- normal ALR screens sacrifice color or view angle to provide light rejection; ultra short throw screens turn the potential difficulties of the severe angle of incoming light into a strength by filtering out any light that is not coming in from such an extreme angle. The result is that you can retain your viewing angle while having near to better than 90% ambient light rejection in many cases. The penalty however is a lower gain thus requiring higher lumens if using a larger screen.
It will be interesting to see how this progresses, because advantage 1 and 3 could see projectors competing for living rooms beyond niche so long as people are educated to pair them with correct screens and budget for it. That's where normal throw projectors are "cheaper", you can get a $200 silver ticket screen with a UHD65 and unless you need the cusp of perfection odds are you will be very happy with things in a light controlled environment. For ultra short throw you really need a good screen/perfectly flat surface or you will likely be dissatisfied. That and cheaper ultra short throw units tend to have poor uniformity.
On the flip side...if no one is watching TV/movies or playing games together anymore then i guess we might as well continue towards strapping our phones to our heads for cheap VR and projectors will fade to obscurity... c'est la vie.
Separate of this, I'd like to slap upside the head whomever in the entertainment industry decided that 3D is dead for home users...somehow none of the new TVs have it, nor does TI's new UHD DLP chip...passive 3D and 4k is such a perfect combo... /sigh