Great post - I've been wondering the same thing recently - hopefully you'll see some good feedback from folks who use these style blocks
I have a similar background - never using anything more than a 17" block through my 35+ years doing this stuff. I also feel the same way that in if you have a low spot that's over 17" you have a big problem!
Personally I feel most people problem with getting panels straight is not letting the block and paper do the work - ie. applying excessive pressure while blocking causing the panel to move...
However, I have a 49 Hudson Commodore that has a HUGE rounded roof and I was thinking one of the longer flexible blocks would be expensive
yet perfect for shaping and blocking this area.