First of all, I would like to say that I'm not a fan of Exo. Don't think I'm delusional just by using them as an example.
To understand, you'll need to click and read the article below.
On OH, Exo's Christmas Day MR removed video is a running joke. But is the removal of the backtrack done properly?
The problem with phase cancellation is that it acts across the whole mix, not just the bits you want it to act on, so if you’re singing along to the vocals on the backing track, then every time the waveform of your voice becomes equal-but-opposite to the out-of-phase waveform of your voice used by some Starcraft nerd to perform the cancellation, your voice gets cancelled out as well.
So it turns out when you do a phase cancellation & the singers' voice hits the same notes as the backtrack, it will be inaudible.
You may think, well maybe the uploader used a different technique - the second one mentioned in the article. If they did a stereo bandpass filter, the vocals would be much clearer and there would not be as much noise.
Tl;dr: We can't use crappy MR Removed videos to judge artists' singing. The only semi-reliable way is to use a stereo bandpass filter, isolating the vocals with a few instruments - which most MR removal uploaders do not do.