But still not THAT hard. A 5th level Berserker in a party of four PCs could easily use up all of his Rages in single fight with a dozen goblins. All that has to happen for him to lose rage is for him to start a turn without seeing any goblin within 30' because they are either (1) hidden from him using their Nimble Escape and Stealth Expertise; or (2) further than 45' away because they were plinking away with their shortbows, and he already killed the one last round that was in range of him. In fact, even if he hits a clump of three goblins and kills one, the other two goblins still can and naturally should Dash away in opposite directions to safety, using Nimble Escape to avoid his opportunity attack.
Yes, there are ways around this, including drawing a dagger and chucking it at the goblins, or even cutting on yourself to keep your rage up if there is no one within dagger range. But having to waste actions and possibly HP to keep rage up is not ideal. And it's also something a heavily-armored fighter or dexterous archer doesn't have to worry about at all.
Low-level Barbarians are okay, but even at low levels they don't eclipse fighters.
