Each has their own unique challenges.

The most obvious challenge in working solo is you have to wear lots of hats. You need to know how to animate, obviously. But you also need to know how to scenebuild, how to light, how to choreograph. And then you have to know how to write. How to 3d model. How to texture. How to video edit. How to do VFX compositing. How to sound design. The only skill you don't really need when working entirely alone is how to direct, because you don't have anyone to direct. You have to be an entire production studio, all by yourself.

Working with other people lessens that burden, but also introduces unique challenges in its own right. The most important challenge is directing. It is a skill being able to communicate with people what you need, how you need it, and when you need it. Making sure everyone knows what is expected of them, and what to do and who to talk to when things go wrong - and things always go wrong. And as the director, you are responsible for everything that happens. If a voice actor or a video editor delivers something that isn't to your satisfaction, then that is your fault, not theirs. You didn't clearly communicate to them what you expected of them. And if it turns out that what you're asking for is just outside of their skill set, it is something they can't physically deliver, then that is also your fault as director, because you made the executive decision to work with them without properly vetting that they can provide what you need from them.

Another challenge unique to working with other people is the release of control. When you're working alone, then you have total control over every aspect of the production cycle. But once you hand work off to others, then that work is out of your hands. You are now at the whims of their production cycle. Bring in multiple people on a project, and suddenly you're now having to manage everyones' time schedules on top of everything else. If it's going to take a week for audio to be recorded, then that means you can't animate lip-sync (you don't have the audio), and it means that your editor can't mix the audio (it doesn't exist yet). And so now you have to organize the rest of the work around that, allotting work that can be done with the material you have while waiting for the material you offloaded to other people.

Ultimately, I find it's a trade-off. Working alone gives you total control over everything, but you also have to do everything. Working with others allows you to focus on the things you know how to do well, at the expense of now your production schedule is caught up in the tangle of other peoples' lives.

There is no objectively best solution. It all comes down to what you consider most important to you and your project.