You have to be subtle with mid/side processing. I try not to use it unless i am struggling tooth and nail to get a little space.
I am assuming you are only talking about M/S processing in general, and nothing to do with recording with the M/S technique.
If using M/S processing in mastering for enhancement and you are doing big EQ moves, your mix balance is probably off. I have stopped using it almost all together, I don't even use it to mono the low end anymore because even high passing the side channel as low as 80-100hz sounds worse to me. M/S techniques can be very valuable to a mastering Engineer who does not have access to your individual tracks but you do.
It don't know why it sounds worse, I can't put my finger on it. Could just be slight phasing? I doubt in reality I can hear much of a difference by high passing the sides at 80/100hz because I mix with a mono bottom end anyway, so the high pass should essentially be doing nothing. but with a blind A/B test I really just do prefer it in bypass.
I didn't watch the video but a lot of people use M/S EQ on the master to duck a little low mid in mid channell, and a bit of air in the side channel, but the sound always sucks for me, the sides become too much, always. Even when pro's show it in videos as an example. If you use ProQ3 which most do then make those EQ points dynamic for more transparancy.
Some people will use M/S to create space in the rhythm guitar for the lead vocal (reduce mid in classhing frequencies to vocal because vocal is more than likely occupying mid channel), but you can do a better job using the more traditional tools that we have always used. There are so many other more basic options. I think with m/s techniques, if you can, always make it dynamic and sidechain making SUBTLE moves.
I asked in the academy I'm in if M/S processing is necessary for a professional mix. the big answer I got back was a resounding NO