I haven't seemed to experience any problems with the MDF frame mould for my boat. Well, nothing I can see with my eye that I know wasn't the fault of the mould (I've made a couple of mistakes, one of which I can see until the hull gets faired.).

Otherwise I agree, for long term build projects, don't use MDF, any humidity will affect the MDF.

For a quick, one use mould (two hulls) such as I have used, I would still recommend MDF, but carefully keep an eye on the alignment.