I’ve ridden in a Lucinda clinic, and most of what I got from it was invaluable. It definitely increased my confidence, and I learned important things to take forward. And my horse loved it that some of the exercises were all up to him, the rider wasn’t allowed to do any steering or rating. Just sit and don’t fall off!
(We were practicing saving it after a difficult jump. That’s a Lucinda exercise.)
That said, Lucinda is definitely not for everybody. Riders and horses need to show up as ready to ride as you’d be for the most important recognized event of the year. She is fairly unforgiving of anyone she thinks is holding back or not really into the exercises. She wasn’t horrible to anyone, just occasionally a bit curt. She expects people to be giving their all and to come with a solid core of basics, even in the lowest level group.
She also had a tendency to fixate on just one or two of the most talented (and expensive) horses and give a lot less time to the others. Even forgot about the others during certain exercises, with all eyes and advice for the one or two favorites. In one session she burned 10 minutes personally changing the bit on one horse, while talking with the parent and rider about where they got the horse and all the ambitious plans they had for the future, while everyone else waited on her. As Lucinda was a 4* (now 5*) rider herself, I understand her interest, but everyone deserved their measure of attention for their clinic experience. Everyone didn’t get it.
I was in one of the lower level groups which was scheduled to go later in the day. I watched a good bit of the upper level group that went first, as I had not seen her clinic before. They were falling off right and left! I was thinking “well we lower-levels won’t have to do what they are doing”. Guess what - every group had the exact same exercises over the same jumps! :lol: (My group did not fall off because our horses were much less energetic about it. None of them bucked like one of the UL’s did.) Her level of expectation of rider skill adjusted somewhat, but her expectation of effort was the same for everyone, regardless of level. 
I guess the shortest way to put it is that it could be the best clinic of your life, or the worst. Much of that depends on whether the rider/horse pair are a good match for Lucinda’s teaching. 