Anyone who knows someone in the entertainment industry in a personal manner (close friend, relative, etc) will understand how they feel when they say they're burned out from being so busy. As viewers, we only see 20-30 minutes of their lives in a vacuum each day.
Arin likely has all sorts of stuff going on. He does voice work, which doesn't only entail going to a studio, jumping in a booth, and talking. There's auditions, demos, and finding the work, which is probably the hardest and most time consuming part. He has mentioned in the past that he oversees animation projects. Not to mention he probably tries to put some time into his own side projects, like Sequelitis. He works on Starbomb when Danny and Brian carve out time from NSP. And then he oversees an office full of grumps, including handling advertising connections for the channel, like Crunchyroll. He likely has a big hand in merchandise development and deals. Not to mention traveling for work, cons and such. While the average Joe sees it as a little vacation for them, they are expected to be "on" any time they leave their hotel rooms when they go to cons. Most fans will want "FUNNY JOKE!" or something. It can be hard to be a people person any time you're spotted out and about. One would argue it comes with the territory, but it's still draining. Then there's his involvement in KKG. All that can leave very little time for a personal life. He's a married man who is clearly very much in love with his wife, and getting more time with her is probably always on his mind. The nature of his work might even make him a bit stir crazy, which leads to wanting more time to get out and unwind, which comes back to being "on" should he ruin into fans when going out. There's a lot to consider.
Then Danny has Grumps, Starbomb, and NSP. NSP is probably far more time consuming for him over the grump gig. There's writing the songs, practicing what's written. Rewriting and fine tuning what's written, but yet unreleased. Concerts are a massive workload, while probably having one of the biggest immediate payoffs in terms of seeing people enjoy what you do. Setting up shows can be gruelling. Danny also strikes me as a very family oriented person. Getting time for his parents, grandparents, sister, etc would already be difficult, then you throw in him living in a different part of the country than them. Despite the Sexbang character, he probably is looking for Ms Right. More fame and success can make that harder.
It's probably draining for both of them from many sides, not just what we perceive on a daily basis from 26 minutes of them playing video games.
That said, I'd love to be in either of their positions, but as a stay at home dad, I can't imagine doing as much as they do and making enough time for my wife and son. Should either of them get to the point of having children (I say this because I've met a surprising number of people who legitimately don't want children), they will find themselves so much more exhausted than they are now. That isn't meant to scare anyone off from having children, as I also can't imagine life without my son.
Basically, they are likely busy as shit and on the whole, happy. Everyone has a bad day or moments where it's hard to see how all the effort is worth it, even when you know it is.
Disclaimer: I made a number of statements in this post that were assumed based on what I've heard the grumps talk about. I also made statements based on experiences growing up with my stepfather who was a comedian. I in no way have any affiliation with any grumps aside from being a viewer, and so my statements are not necessarily representative of a day in the life of a grump. But I wouldn't think they were complaining, as much as being candid with their fans. For me, then being candid and honest is when I find them most interesting.
Just don't read so much into it. We all vent. Grumps are people, too. :)