I honestly don't think FADEC is an issue at all for training. There are far more differences between e.g. a DA40 and a typical school plane, not least that a DA40 will have a lot more stuff in the panel and, being a new aircraft, the stuff is likely to work!
I did my PPL in C150s, PA38s, C152s and while all of these had a mixture lever, nobody ever told me what to do with it, other than shutting the engine off with it.
And if someone has done their PPL in a DA40, they will never want to get inside, let alone fly, anything else that is normally on offer!
Last time I spoke to Diamond the diesel DA40 was listed at about 160k + VAT so I don't think everybody will be worrying about this anyway
