Derate is quite different from flexing, at least in the way boeing uses it on the 737NG, there's no derate i know of on the A320 although the A330 seems and probably other airbus types seem to have some kind of that.
Anyway, derate is the same as using another engine for that take off, it includes different limiting speeds and although you can use full thrust, you shouldn't do as you will go outside the certified envelope for that derate which can result in loss of control. Especially the lower VMCg with a derate can actually improve take off performance on a contaminated runway, as one can use a lower V1.
Flex on the other hand is still using the same limiting speed as the thrust rating used. One can legally advance thrust to the limit of that rating (which might be full thrust if flex and derate are not combined). As far as i know there is no certification point for use of flex on contaminated runway. Therefore it is not allowed by the OEMs as it is outside of what they test and certify for.
But there have been quite a few threads about that issue in the past on here.