Here's how I see it:
Early EMH parks will be more crowded by mid-day than the same park will be on a day that does not have early EMH. Many onsite visitors interpret EMH as "Disney is telling me where to go so I'd better do it" and also as "Hey, more hours in the park is a better bang for my buck!" What they don't realize is that more hours in a crowded park is equivalent to fewer hours in an emptier park... You're just in longer lines all day.
Your aim should be to squeeze the maximum benefit from the early EMH hour, or else avoid it entirely. No in-betweens.
The "avoid EMH parks completely" blanket advice you've seen is for two sets of people: 1) people who stay offsite and will thus not have access to the early entry anyway and 2) people who stay onsite and will not get to the park in time to enjoy the entire early entry hour. In both cases, these sets of people want to avoid EMH because they don't want the downside of extra EMH-related crowds without the upside of that extra hour of riding in a semi-empty park.
The offsite people might choose to avoid EMH because, by the time they can get in the parks, a whole hour's worth of onsite people will already be in there, in ride queues. Why not just pick another park, where you can be one of the first people in the gate and on the rides?
The onsite people who "waste" EMH by arriving at 8:50 for a 9:00 general opening have given up most of the advantage of early arrival. So they've missed almost the whole hour of low-crowd riding, and now they're in the park that has an attendance surge from EMH. Even worse, sometimes onsite visitors show up at the park after EMH is over and miss the entire benefit of lower crowds. Very, very poor planning.
If you stay onsite & can be at the park prior to EMH opening, it is a GREAT time to ride lots of things with a very low crowd. It's even better if you have parkhoppers so that you can switch to another park later if it becomes too crowded.
Just realize that morning EMH hours does draw more people into the park than non-EMH days, so later parts of the day will be more crowded. You can either accept this because you value what you can do in that bonus hour in a semi-empty park, or you can parkhop to a non-EMH park later in the day. Or, of course, you can avoid EMH parks entirely.... it's up to you!
I just hate that "always avoid EMH, no matter what, under pain of death" advice. Because... no. If you're going to FULLY UTILIZE it, it's a fantastic time to be in the parks. And especially if you can hop later to an uncrowded park.