Complete nonsense.
I've had both screens side by side when I purchased two MacBook Retina 15's.
Both screens look identical in brightness, color, tone, and every conceivable way. I checked this with a light meter and a color spectrometer ($1000 tool for calibrating cinema displays) and both showed under 3% difference in brightness and even less in color with the Samsung being darker (a 3% difference is negligible in brightness and you would never tell).
The source of the problem is likely all in your head.
The LG displays have also been fixed for burn in (my first LG display did have severe burn in though) so there should be no complaints.
I had an LG display with image retention, so I measured and calibrated it with a spectrometer before taking it in, and did the same to my Samsung replacement screen.
Brightness wise, the Samsung screen is exactly 1/4 of a bar brighter, hardly a noticeable difference. The LG is 120 Lumens at 11 1/4 bars and the Samsung is 120 Lumens at 11 bars. As for colour, the two screens are completely different. The Samsung is much more yellow, so when I load the LG's profile on the Samsung screen, it looks very yellow. The original Color LCD profile looks very green on both screens (most would say a properly calibrated screen looks warm/red).
The yellow of the Samsung doesn't bother me at all as I calibrate my screens, so they essentially look the same once calibrated. Unlike the yellow on my 27" iMac, the yellow on the Samsung screen is consistent over the whole screen. so it's easily correctable through calibration.
EDIT: FYI when they replace the screen, they also replace the top case and the battery. I had some dings on my case (near the speaker and on the edge where the cables plug in) and 130 cycles on the battery. All fixed with my new Samsung display. Quite happy right now!