For a breakfast bar (say) I prefer not to have the surface completely unsupported, even if the spec will allow say 300mm. You get people plonking children or even heavier beings on them (eg women), rather than just balancing a breakfast bowl and some glasses. When selling a property as a developer you cannot realistically sell with a list of "don't do this, that and the other" and it is cheaper to play safe at the outset and anticipate stupidity or carelessness, than deal with a warranty claim later. Replacing worksurfaces on a finished site is a right pain and takes up management time as well as site team time. Corian needs to be really well supported and in my experience with former clients is a poor choice for large overhangs unless you have built a really solid cantilever frame or some such. I well remember an issue with one client who had a moulded in sink and it distorted from her frequently pouring boiling water into it. You are not supposed to do that and it meant replacing the entire top.
The local bespoke maker where I live now will do 300mm unsupported on quartz (and whatever you like supported. In my own kitchen, build (by me) currently in progress, I will overhang the quartz by 30mm or maybe 40mm - I will decide when it is templated next week. This is all built on a very solid oak frame, all the bespoke internal divides are 25mm baltic birch ply, and the top sits on this and the rebated oak frame. The top on mine is 18mm ply and this will support the quartz top, which has to be in two pieces because of the size. Cuts for induction hob, tepinyaki and downdraft extractor are all supported from below.
I prefer quartz these days, especially if using 20mm for elegance, because it is so tough. Corian does not like hot pans, it does not like big temperature variations, it scratches easily and in my experience determined clients managed to stain it as well.