hmm... what is the debate about concrete? I just replaced all of the plastic pots in my pond with concrete. My situation may be different in that my pond is also concrete without a liner. I'm guessing that the debate centers on the elevated PH that can result? I decided to go ahead since the PH in my pond is still so far over 9 that it is not readable with my test kit. Plants and fish are doing fine now so I'm not worrying about it anymore.
The only problem I could see with clay pots would be that they can break down over time in water, especially if they are the plain clay ones, not fired at high temps.