Honestly, both are hard. Just hard in different ways. And if you asked me which one cost me more nerves, I'd probably say: finding the right people.
Writing a book is solitary work. Hard, slow, sometimes impossible, but solitary. Just you and the page. You decide everything. Everything depends on you. Nobody's late, nobody sends you files in the wrong format, nobody goes on vacation at the worst possible moment.
But after that, a completely different story begins. I didn't have a ready-made team — I naively assumed at first that I'd find one place that would hand me everyone at once: editor, layout designer, illustrator. Nothing of the sort. I had to search for each specialist separately, on my own, by trial and error. First one, to see if I liked working with them, to look at the result. Then the next. Then another, because the first one wasn't a fit. I looked for a translator separately from an editor, an illustrator separately from a layout designer. I'm still looking for a literary agent, and that search shows no sign of ending.
Every one of those searches costs time. And if I got the choice wrong — which I did, more than once — it cost money too. I paid one editor for a trial chapter, it didn't work out, on to the next one. I commissioned an illustration from an artist who couldn't pull it off — money spent, no result, start over with someone else. Nobody tells you in advance who'll deliver and who won't. You only find that out in practice, after you've already paid.
An editor isn't a contractor. They're a partner with their own opinion. A good editor will argue with you, and that's exactly as it should be. But you have to learn to tell a substantive disagreement apart from someone just pushing their personal taste. A designer lives in a world of images, not words — I'd come to them with descriptions and get back something completely different, sometimes better than what I'd pictured, sometimes not. Every specialist has their own language, their own idea of what 'urgent' and 'done' mean, and you have to relearn that language every single time.
After two books, here's what I've learned: the hardest part isn't working with a team — it's that there was never really a team. There was just me: author, client, and the person who did the searching, the vetting, the making of mistakes, and the paying for them. Three roles at once, and nobody had assembled any of it for me ahead of time.