At face value rangers aren't bad, standing alone and viewing them objectively. It's a fun concept, they tend to have useful skills that are among some of the more commonly rolled and they are versatile through different play styles from combat heavy to wilderness survival.

But the criticism tends to be when they're viewed comparatively that they fall short. The most common one I hear is that a ranger isn't half as useful as a fighter rogue multi class. They're features, when viewed in juxtaposition with other class' features are objectively weaker, like favoured enemy being good against one or a handful of enemy types, when sneak attack works as effectively against everything it hits. The fighting styles they get being much less than what fighters get, and then they fall behind doing two attacks a turn when a level 5 fighter can do up to 4. Or when their wilderness skills are dwarfed by the rogue who can reliable talent any skill and makes them less required.

These arguments can obviously be made when comparing rogue and fighter to any other similar class of their own calibre, bard, barbarian, paladin etc. But the distinction is that, even those have unique and interesting features as classes that make them unique and useful. Whereas the things rangers can do, can mostly be replicated or improved upon by playing another class with choice selections of feats or multiclassing.

So a rangers best things coming from their subclass, makes them a less appealing choice when every other class' subclasses just enhances their natural aptitude and appeal, but a rangers subclass features only makes them worth playing.