As a standalone game this would probably be 3 stars, there are issues such as things being unlocked at the very end of the game after they've become irrelevant, and the fact enemies don't respawn makes it nearly impossible to grind up more pikmin if you want to.
As a Pikmin game they have undone many of the things that made the previous games excellent. The lack of a time limit in particular makes the game boring, as it simply lacks incentive. It also seems to contradict much of the storyline. A large part of the fun of pikmin is having to go back to old stages and explore them with *new* pikmin. This is simply never necessary in the fourth game. You never have to go back to a previous stage for any reason. When I played, I made it through 100% of each map before I moved on to the next. I won't speak about difficulty, as far as I'm concerned the game is fine on that front, anyone who wants the game to be punishingly hard is an elitist. Pikmin has never been difficult, and it has never seemed like it's supposed to be.