Pull what said:
Chris
Thermals are only one of the three ways that Cu is produced, a thermal is a convection up draught.
Downdraughts only exist in the mature and subsiding stage of the cloud, in the initial buildling stage the cloud is composed of updraughts which may be caused by convection, orographic or frontal lifting.
I regret that your statement that downdrafts only exist in the mature and subsiding stage of the cloud does not fit with my experiences over many years of gliding. Taking thermals, there is sink (downdraft) all around them virtually always - even before they have risen to a height where condensation occurs and cloud forms. When the cloud forms the thermal rises more rapidly (wet adiabatic lapse rates & all that) and the surrounding sink does become stronger. At a later stage when precipitation has occurred the downdrafts increase in strength and are usually strongest on the upwind side of the cloud.
A risk in flying in or near very large cu / cunims is that they can grow in size much faster than you might have been expecting and what you thought might be 5 minutes in IMC could end up as 20/30. They can also get very rough making handling difficult - even ignoring the fact that you are on instruments.