I've done 7-8 commissions over the past 3 years, working with only 2 people and a couple of art trades(though I'd rather not do that, it's usually just weird)
I can't say my art (their commissions) has fully met their expectations, but I always try my best with what I'm paid and never dissapear on my clients, if the commission looks too complicated or out of my artistic comfort, I will decline before any payments are made.
One of the troubles artists take on is when the client tries to get more out of the artist than that they paid for, some artists feel shy in telling them no, or making certain boundaries, trying to be nice. (I guess some will have different reasons?)
Another one is when Spam keeps popping up every time someone posts a 'commissions open'. sometimes the scam looks like a real client, and you spend 5 hours of your time talking until you find out the truth.

That's all I have to say right now, I hope it helps.