The latest major expansion for Path of Exile is indeed considered a success, bringing even more dimension to the already stacked online action role-playing game. However, the quality of such a big content update isn't just in what it added, but also the way it changed how the game is played. Developers generally have good intentions and set plans for the future whenever they put out something like an expansion to their game, so it must be assessed whether Ascendancy did indeed accomplish what it was set out to do.
Ascendancy was aimed to create more diversity in the game, but such a quality isn't just about the number of options available for players. The most important factor is how many of those options are actually viable in the game. If there are some of them that pale in comparison to the others, they'll be mostly seen as non-options that wouldn't be recommended to be taken by players. It does seem like it's what a lot of players are worried about with Ascendancy.
Path of Exile is all about being able to come up with your own way to play the game through coming up with unique character builds and variations of existing ones. It's that kind of flexibility the game banks on, and Ascendancy was supposed to stretch that even further and provide an unprecedented amount of choice in not just the game itself, but the whole action role-playing game genre. However, as such lofty ambitions tend to be faced by, it may not exactly be the case.
Some have been thinking that even with all the Ascendancy classes and PoE items added to the game, the number of viable character builds that can get players all the way to the end game in any league have not really increased that much courtesy of the expansion. They'd argue that while the gimmicks with the Ascendancy classes and all the nodes in each are indeed quite interesting and can potentially bring about new playstyles in the game, all that haven't really lived up to high expectations.
Perhaps there have been some more builds that indeed became viable in the game after the introduction of Ascendancy, but the further specialization brought about by the Ascendancy classes have merely divided the potential of each class. Coupled with the removal of being able to branch off to a different class on the passive skill tree, it may seem like Grinding Gear Games had hit reverse when it comes to the game's original design philosophy.
More character build variations can then be created through addition of more unique Path of Exile items that can be built around, but perhaps most people are taking the whole thing the wrong way. Builds and diversity aren't really as important as you might think anyway as Path of Exile isn't supposed to be like most other action role-playing games anyway, and the design of Ascendancy is supposed to amplify that. Think of the starting classes not as set paths, but merely guidelines that may be steered to other directions whenever you please.
That's the point behind Path of Exile's game design in the first place, so don't think of builds as be-all-end-all set of strategies to set your gameplay in stone. It's not like serious PoE players would stick to just one character anyway, which is why the additions that Ascendancy brought in are indeed good. While there may indeed be pigeonholing involved with the Ascendancy classes, but that only adds to the amount of things you can do in the game anyway.