True that. Maybe we need to get a "Red Flag" list going. Once we take on a new client, two things can happen: Things go well or they don't. And in the "Doesn't go well" category, we have 2 buckets: We part ways, get paid and we move on. Of course, there is some anxiety, frustration, time loss in these situations, but at least we keep it to a minimum. The other bucket is when things can get dangerous and more serious...allegations of malpractice, non-payment, etc. And these things get dragged out, more time wasted, etc.
I think you're very wise to evaluate what this client is "telling you" not so much with the words, but with your characterization - a complainer. Often, these folks turn out to be very difficult clients. And the new preparer is just the next in line to receive the abuse. Also, as part of your comment, consider the obvious fact that the person is changing preparers. Preparer jumpers are a bad sign to begin with...and this can be ascertained by reviewing the preparer info on the last few years' of tax returns.
In some cases, however, complaints are reasonable...and if told to you in a non-blaming matter, unassuming manner, and with an even tone, things might be fine. Had a call the other day, "Hey, Chris. I'm with a big firm now, but I don't think I need to use them anymore since my business activities have wound down significantly over the years. They also change my preparer each year and I feel like I'm just their playground for new trainees. And, their fees haven't gone down although the complexity has."
Fair enough. Reasonable arguments in a reasonable tone.