I can only speak from experience, so I will. Here are the reasons I found him to be so frustrating, which all sort of come down to poor communication on his part:
If I sent him an email asking three questions, at best I could expect half an answer to one question at a time, if I heard anything at all. I would have to nag him over and over if I wanted to ever hear an answer to anything. I am talking at least three emails and asking him over a messenger before hearing "the sketch is okay, but make the feet bigger." If you don't harass him, you will probably not hear an answer. For example, when he first emailed me, he basically said "I'd like you to draw my Hexadragon" and I replied immediately with "here are my prices, what kind of commission would you like (i.e. sketch, digital drawing, etc)?" I never heard back from him, so I assumed he was no longer interested and I went on to other commissions. A FULL MONTH later, he emails again saying "what's the deal? Aren't you going to take my commission?" He still didn't tell me what kind of commission he wanted until I pestered him again.
He is not clear about what he actually wants. In my case (and everyone I know who has taken a commission from him) he asks for YOUR interpretation of a Hexadragon. He will give you some guidelines about the numbers of eyes and legs and toes and so on, but tells you that all the other details (such as coloration, markings, spikes, wings, whathaveyou) are all up to you. Sounds fun, right? When you sketch up a rough draft and show it to him, he will tell you to change stuff. "I don't like the face." "I don't like the pose of the tails." "Make the feet bigger." He will tell you to make it more generic. More cliche. More like a traditional dragon. So actually, he's not really asking for your ideas at all. He is asking you to draw an idea he already has in his head, but he won't tell you what it is. I wound up having to start my drawing over from scratch three times because he didn't like them (though for the record, one of those times was because I misread an email and thus was my own fault.) Finally I just drew something really boring and he was satisfied.
In the end, with enough nagging, I DID get my payments, so it's not like he ripped me off or anything. And for the record, I'm sure Ksilebo is a great guy, and I'm sure there are some artists out there who have had a fantastic experience with him. Personally, though, I wish I had never taken his commission because the irritation alone wasn't worth the price of the commission in the first place.
My advice to anyone who hasn't been commissioned by him yet is either refuse him or make sure that the price is high enough to compensate you for all the time and effort you will waste just trying to get some clear information from him. Be prepared to wait at least a week, maybe a month before getting payments or replies. Make sure you establish in advance that you will only make changes to the drawing x-number of times before charging a fee, or that you will charge a fee per change and a larger fee if he makes you draw it over again. Also, don't bother being creative and save yourself a lot of heartache by drawing something boring from the beginning, because apparently that's what he wants.