The short answer is yes, they improve the game, and are worth using.
Unchained Summoner—a justified nerf
The original summoner was extremely powerful—the eidolon could easily be as powerful as a whole ’nother character, plus the summoner’s summon monster spell-like abilities on top of their very-good spell list, made them an easy candidate for the upper echelons of power in Pathfinder.
The unchained version, unlike the other cases, is a nerf—and a well-justified one. It tones down the eidolon somewhat, and removes some of the higher-level spells that had been forced into lower levels so the summoner could cast it (which turns out to be largely a benefit). That makes the unchained summoner a much more balanced class.
Note, however, that the original summoner was not the most powerful class in the game. Clerics, druids, shamans, witches, and wizards were all more powerful. A given oracle or sorcerer easily could be. And none of those classes have been nerfed. In a game where those classes are being played to the hilt, the summoner should perhaps remain “chained.”
Unchained Monk and Rogue—massively necessary
These two are the ones that were really “unchained,” that is, powered up—and they needed it.
Everything the chained rogue can do, the ninja or vigilante can do better. Even if we ignore the unchained variant, there is just zero real reason to play one: if you want to play a “roguish” character, just use ninja or vigilante to do it instead.
And the chained monk... the chained monk can’t really do much of anything. It’s one of the weakest classes in the game. The whole concept of “mobile, mystical warrior” can be done better by... most of the classes in Pathfinder, honestly. Alchemist, brawler, cleric, druid, inquisitor, investigator, oracle, paladin, and ranger all easily replace monk for just about any character that might take the class. Even fighter, ninja, rogue can do it, and probaby do it better.
Both unchained monk and unchained rogue end up in a similar space: light, fast, flexible strikers par excellence. The unchained rogue is probably the best straight-up damage-dealer in the game, and has lots of skills. The unchained monk doesn’t quite match the unchained rogue in damage, but its damage is still very good, and a number of its mystical abilities allow for more versatility than the rogue can muster (at least unless the rogue is using magic wands with Use Magic Device, which every rogue should, but that does require planning and it does get pricey).
Unchained Barbarian—I honestly don’t know
I had actually forgotten that the barbarian also got unchained treatment. The chained barbarian isn’t in an awful place like the monk and rogue, but it’s still a little lacking. Rage powers are nice but tend to have qualifications, limitations, or just be too small for the number of rage powers you get. So I think there is some room for improvement. But I don’t know the unchained barbarian well enough to say whether or not it represents that improvement. On a read-through, which is not the same as play experience but quite possibly worth something, I tentatively agree with Ifusaso’s analysis on the unchained barbarian—it’s not really better, or worse, just different, kind of. It doesn’t look like the improvement I would have been looking for.
Replacing other classes
As for other Pathfinder classes and obsolescing things by unchaining these classes... yes and no. There are classes that are better than the chained monk and rogue, but not as good as the unchained monk and rogue. The fighter probably falls in that category, for example. And the summoner, unchained or not, can replace a whole lot of other characters—in some cases, the summoner’s eidolon pet can replace other classes wholesale (e.g. the chained monk and rogue, for sure). Pathfinder is not a balanced game. It’s not even close to a balanced game. That’s just reality, and without redoing every class—rather than just four—it isn’t going to change.1
Moreover, classes in Pathfinder are quite amenable to “refluffing,” that is, keeping the mechanics the same but replacing the descriptions of them. Monk characters that are actually using the barbarian’s class features, calling rage “zen focus,” for example. Or clerics taking Improved Unarmed Strike, and calling their spells ki instead of prayers—that is a good replacement for monk, even unchained, since cleric is one of the strongest classes in the game. Since the chained monk was so bad, these kinds of replacements for monk were common—and commonly recommended. Unchaining the monk makes it a better choice, less needing replacement—and, in turn, may make it a solid replacement for other, weaker classes. I could see using unchained monk to create a barbarian—what kind of maniac goes into a fight with no armor, anyway?
But what it basically comes down to is this: there are classes better than the unchained barbarian, monk, rogue, or summoner. There are classes better, even, than the chained summoner, though fewer of those as it was quite good. All of the classes that got the unchained treatment—even after unchaining—could reasonably be replaced by a more powerful class able to do what they do, but better or more.
Unchaining the monk and rogue moves them from the bottom of the pack to somewhere in the middle. Unchaining the summoner moves it from head of the class to also somewhere in the middle. Other classes at the bottom of the list, that aren’t unchained, now have less competition for “just the worst,” but frankly they were always poor and at least unchaining the monk and rogue gives more options that are good.
See the Pathfinder class tier list for more details about what classes are weak and what classes are powerful, and why. You’ll see that the unchained monk, rogue, and summoner are in good company, right in the middle of the game alongside a large number of other classes, including most of the hybrids. Do some classes get left in the dust? Yes. But they already were. Unchaining just moves the monk and rogue out of the dust, and takes the summoner off its pedestal. (The unchained barbarian is not addressed, because again, I am not particularly familiar with it.)
- Many people thought rebalancing all the classes was what Paizo was going to do when it created Pathfinder from D&D 3.5e in the first place, since one of the claims made was that it was going to improve balance. But that turned out to be just hype—several of the weakest classes in Pathfinder are distinctly worse than in 3.5e, rather than better, and several of the most powerful classes have gotten much more so.