I didn’t see if anyone has specifically mentioned this yet, but you only need to go to one regional training session to qualify for nationals. I’m not sure about the other qualification requirements as they’ve changed quite a bit since I did EAP in 2012 which feels like forever ago!
Like other posters have said, your experience really depends on who the clinician is. I had the privilege of riding with Kip Rosenthal, who sadly passed away recently, for my regional training session. She was absolutely fabulous. I learned so much and never felt belittled or ignored. That being said, I did notice they grouped the riders by skill level and I was in the highest skilled group, so other people may have had different experiences. That was the first thing that bothered me about the program. They started with one set of groups the first day, then changed them up the second day. They claimed it wasn’t by skill level but it was pretty obvious that it was. Everyone knew it, why not just be honest about it?
I can remember one instance of someone being called out in front of the group. She didn’t have her long hair up and one of the organizers (not the stable manager or clinician) told her off for how dangerous it was. I can see where she was coming from, as it can be a safety concern. And we can all learn from each other’s mistakes. But there’s a way to bring attention to it without shaming someone. It definitely left a bad taste in my mouth, but I can tell you I haven’t been around horses without putting my hair up since that incident!
Our stable manager was fine for the regional session. He didn’t teach us anything groundbreaking, but lots of good basics. A not insignificant number of other participants didn’t know how to do basic things like tie haynets or properly deep clean a stall, so it made sense for him to focus on the basics. My horse was kind enough to provide another on-hands learning experience for everyone when he tripped on the first day and needed stitches in his knee.
So we got to see up close and personal how to use a spider bandage. Luckily I didn’t have to use one overnight since I don’t think I could pull one off! But I did have to do lots of extra wrapping over the weekend. (My horse was cleared by the vet to continue for the weekend. Looking back, I probably would have called it a day and scratched, but he didn’t suffer any long term issues from it so I don’t regret my decision. I’m just a lot more paranoid now.)
Maybe all the extra wrapping helped me, but I got selected for the national training session. I seriously wasn’t expecting that! My horse wasn’t an easy ride and I’m certainly no eq rider, but I must have done something right.
The national training session was pretty cool too. I got to ride an amazing, albeit difficult, horse, and learned so much in the stable management. To be honest, I didn’t get a lot out of the riding portion because I had trouble with my horse on the flat. It was hard to do some of the exercises since I wasn’t able to get him to do basic things like extend/collect and bend properly, etc. Looking back, It was probably partially my fault, but he was definitely a jumper and didn’t have the finessed flat work I did with my personal horse. People that drew more eq type horses had a better go of it. I felt like Peter Wylde definitely focused more on the people that didn’t have as much trouble as I did, though he did get on my horse and work through some things. It wasn’t a negative experience, but I definitely learned way more from Kip. I wish I had the chance to ride with her again. She was such a good teacher. Positive but didn’t sugar coat things. Even though I only rode with her over that one weekend I miss her.
It was an interesting experience for sure. I’m glad I did it. I learned a lot, mostly from Karen Golding in the stable. We learned strapping, how to make our own poultice out of bowie clay, how to properly bank the walls in a stall, and lots of other little tricks (try cleaning the outside of hard shell Eskadron boots with rubbing alcohol - it’s like magic!). We also had a great talk from one of the physiotherapists of the Olympic team about equine anatomy.
@Rel6 as someone who suffers from migraines as well I’m impressed you were even able to ride through your migraine! That takes some serious gumption. I’m sorry you had to go through that. As far as giving the clinician a heads up about your migraine, I feel like it’s kind of a “damned if you do, damned if you don’t” situation. Your very valid health problem could have been seen as an excuse for your “poor performance” and you could have been belittled even more 
Overall, OP, I would say give it a try! It was a great learning experience, and not just about horses or riding. I learned a lot about the type of person I wanted to be (and what I didn’t want to be). If you do well, it could potentially open a lot of doors for you in the industry, if that’s your thing. It’s a great way to make connections with some of the bigger names in the sport for someone who wouldn’t otherwise have the opportunity.