The 3.0 TDI was multitronic (i.e. CVT) and the 3.0 TDI quattro was s-tronic (i.e. DCT). I don't know much about the multitronic but we do see the occasional problem report on the forums, particularly with older cars, so it is clearly not a trouble-free transmission. The situation with s-tronic is more complex. There are sufficient problem reports from owners of 2009-2010 cars to warn against them. 2011-2013 is better but there are still reports. Audi introduced the 3rd version of this gearbox in around May 2013 and so far there are no significant reports of problems with this latest version. As
@V6Tom advises, don't buy a car which has missed its gearbox service, no matter which gearbox.