I have watched Philippe Karl teach and ride in person and he is the epitome of excellent equitation. He can get on any warmblood, tb, whatever and make it better by the time he leaves its back. But with that said, I've seen others emulating him who clearly don't have his depth of experience. You just can't replace years of apprenticeship at a place like Saumur with the guidance of elders and the libraries to read with a three year, a few weekends every few months education. The education he received that he is trying to pass on very nobly and precisely unfortunately won't take hold partially because of how many people do it wrong and post videos of their inverted, above the bit, off the aids, hollow horses. That is NOT what Philippe Karl himself does.
So having ridden with one of his certified trainers and with a few people from the Spanish Riding School, I had very similar lessons with equal results on both sides. I ride and host clinics most consistently with a Bereiter from the Spanish Riding School because I really love his approach, his enthusiasm, and his tact with all of the riders in the clinic.
Just my humble opinion...I would be HONORED to ride with or have Philippe Karl or the Bereiter from SRS teach me or ride me horses ANY day!!!