I am one of those puzzle fans you mention in the first sentence that cannot get enough Picross.
For me, this game is a 10/10, but i appreciate the points you make in the review.
When you first learn how to play Picross, you learn some basic rules to help you solve the puzzles.
As you continue to play more and more, the basic rules become ingrained, and you notice other more subtle rules.
An you continue to play even more, all of this becomes automatic to some degree.
Mega Picross was a nice change in that regard.
It forces you back into the early days of learning the game, when it was more difficult.
One other thing I will say is that i have purchased many iOS Picross clones over the years.
They are never as fun, and confusing to play[for me], as compared playing Picross with a d-pad.
I use the d-pad to count squares, but i can't do that on iOS.