Its
insanely dangerous. Really just neigh-criminally stupid.
Any little lip in the siding on the wall could have caused him to flip or twirl about or get speared or launched his engine all over the place. The walls are designed to keep people safe, including the fans. They are fantastically well engineered and made. But, if he had caught the wall wrong and was catapulted into the stands, he may have killed a lot of people, let alone the danger to other drivers, let alone to himself. The various safety systems, of which the wall was but one, aren't made for that move and could very easily have been compromised in very bad ways.
I mean, it was awesome to watch, just incredibly cool. I'm glad it worked for him, he's in the history books for that move for sure. I can totally see NASCAR evolving to utilize that move in the future.
But he put many lives at more risk than anyone was expecting in such a kinetic sport.