It irritates me anyway that ERT says smth like this although he usually proclaims theories of the chinese coachings, and they most certainly do not start with the fastest setups, at least from the knowledge i gathered in the last years.
What annoys me the most currently about ERT is the fact, that he makes videos and posts about the equipment of the pros, using information from tabletennisreference which is in 99% of the cases either plain wrong (the pro never used the rubbers or/and blade) or long outdated (the pro doesnt use the mentioned equipment anymore). Either way his information is at least questionable.
Besides the points Carl and Tony already mentioned, i think you definitly CAN learn a lot from youtube coaches, at least in terms of the basics and certain ideas and techiques.
For simple matters i actually kind of like the videos of Ti Long and tabletennis review. For more in depth i highly recommend pechpong, especially because here you can learn from someones own experience in his own game and see if it fits yourself too.
I second that I follow Ti Long and pechpong as well. They are much more helpful than ERT.
However, once you understand table tennis and once you learn the basic strokes, everything else is a variation of these basic strokes. Once you watch a couple video's, you should be able to extrapolate.
ERT's channel is entertaining but in more recent years, it has become a click bait channel. I mean for an 18-minute video, you will be watching 16 minutes of the same stuff being repeated over and over again. I am glad on my YouTube algorithm, ERT's contents are appearing less and less.
A coach who is good for advanced player might not be good for beginners. The opposite might be true as well: a developing junior player might outgrow his or her current coach at some point.
That is the case across the the world of sports and music instruments (such as violin teaching).
To me coaching is intuitive. You just need to assess the situation and work with what you have. Many adult players who begin their table tennis journey have certain limitations (back pain, elbow issues, etc.). A good coach knows how to work with these limitations and give their students the most enjoyment out of table tennis.
As for the developing junior players, they are not all slate of sand where you could just impose whatever techniques you have on them. Some of them are more intuitive with certain strokes (I think there was a reason Manika Batra became a long pips blocker on the BH side). Some of them have this level of motor or a level of touch (or feel for the balls) that others don't have. You work with that. Some will progress quickly. Some might progress slower, and the coach's job should be to keep the sport fun and encourge the kid to use what their learn in table tennis to become successful in other areas of life.
But a good coach is able to assess the situation quickly and already have the next 10 lession plans laid out.
One time, I was at the club and an adult player was getting lessons from a junior coach (junior as in young, inexperienced coach who is a very very good player). They were getting frustrated and both seemed to be clueless about what was going on. I took a look and saw the adult player was hold the penhold blade way too loose/shallow and his wrist was moving all over the place. I corrected that and he still thanked me 3 years later (I used to play penhold). It was a simple easy fix but both people at the table were not able to see it.
Another adult player I was playing with in double, he just kept on missing all the loops coming at him. I finally got frustrated and told him, get close to the table and just block the top spin right off the bounce. No need to smash the balls. No need to even freaking time the ball. Just take the ball right off the bounce and loosen your grip. Then immediately, he started blocking all the loops back 90% of the time. He later told me, well, I look at other advanced players in the club and they are all countering topspin with more topspin so he thought he should do the same. I am like, no, you develop the touch first and then you can graduate to smacking the ball as hard you can later.
A lot of the adult players at my club never got proper coaching. I had to ask them to come closer to the table and hit the ball less hard to "feel their blade." Table tennis is a sport where you develop the touch close to the table and then gradually you move away from the table using the same strokes you develop before, but with bigger motion and faster speed. Apparently that basic concept is missing (well, maybe they all got started on Timo Boll ALC with Tenergy 05 as their initial set-up so.....).
Let us say, coaching is also a talent. Some people are talented in coaching while others are just not.