Hi,
Yes, I have a BP that I actually still use sometimes as an external monitor for the Inspire via HDMI from the TX.
I use the Nvidia Shield for the Inspire but have made myself a 'Frankenstein' sun shade of things I had laying around in hobby boxes etc. I stole the Hoodini idea of utilising a 70 degree angle between the shade and the screen and it works perfectly.
FYI, the sun is not the issue in viewing tablet screens outside....... It is the glare. Give it a try by going outside in sunny conditions and taking something like a magazine and putting it over the top edge of the screen at right angles (like 99.99% of all sunshades).......it's pretty useless, now angle the magazine down to 70 degrees and see how nearly all the glare is eradicated and you can view your tablet comfortably even in midday sun.
OK, so, back to the BP and it's screen........ I think it's a combination of why that one works outdoors.
Firstly, it was designed to be an 'outdoor' device specifically for FPV
Secondly, it uses LED technology which differs from the mostly OLED screens which most tablets are now - Yes OLED is supposed to be better but..........
Thirdly, the BP is not trying to be a high definition screen so the manufacturers are able to play around a bit with how the backlight and reflective cells polarising filter etc are designed (although Flysight probably used an existing panel after testing many different types).
Fourthly, you are correct...the BP has matt like outer screen on it which seems to fair better in outdoor situations.
I have to say though..... Now I have 'Frankensteined' my Shield I will take viewing that over my BP anyday!
Using the Inspire with a tablet is fine..... Thousands are every day in sunny, overcast, snowy and beach conditions. You just have to be selective/creative with some sort of shade and your FPV experience will be awesome