We were struggling with whether to plan a visit to Skipper's Canteen on our upcoming trip, too - the reviews have been all over the map.
One common theme is that the SC has a few fixable structural problems (hit or miss service with long waits that some claim stems from a small kitchen, unbalanced theming between the main room and the smaller side rooms)... but I assume they're actively working on that operational stuff. That's not the main issue.
My theory on why the Canteen hasn't been as well received is that Disney is catering to a bunch of really diverse audiences in the MK: people who expect hearty portions of comfort food for their money and picky eaters (both child and adult) are a force tugging them in one direction, and adventurous diners who are looking for something more unique and exciting than an average theme park experience are the opposite.
I think the main source of controversy is that Disney tried to split the difference between both of those audiences and ended up satisfying nobody completely.
On one hand, I've read various complaints that the menu is much too adventurous (unexpectedly foreign/spicy without tame options, only side choice is broccoli), portion sizes are inadequate, and that the kids menu doesn't have anything familiar. On the other hand, I've seen people who enjoyed the menu and plating - but complained that the dishes aren't well executed. For those who eat ethnic foods regularly at home, they find Disney's version isn't living up to their favorite restaurants.
I suspect there's truth on both sides of that equation. All of that aside, there seem to be a few favorites emerging on the menu - in particular, the shu mai and the arepas seem to be well loved from the appetizer menu. So it's not like there aren't dishes worth the visit - you just need to choose carefully. We're debating making a meal out of multiple appetizers, though - maybe splitting a main as well to go with it.
We ended up deciding this is a must try for us despite the reviews - it's the most interesting/different lunch table service option in MK, and we prefer to sit down to a relaxed meal during the 1PM-3PM afternoon doldrums when lines are long and kids are cranky. And we really want to support the idea of more adventurous cuisine in the World - especially since we know there are so many great undiscovered dishes out there that Disney could introduce people to on vacation!
I do share in everyone's frustration that they aren't offering ADRs except same-day: we would be pretty frustrated if we get stuck with quick service lunch on our Magic Kingdom day!