Since I'm in Zone 10a, I can tell you that for many years, you couldn't find a Clematis at nurseries around here...which is baffling considering the American Clematis Society is very close to me.
Even when I worked at nurseries, we were to tell people that they wouldn't grow here...so obviously, we didn't carry them. IF in fact nurseries did carry them, they were few and far between.
Once I even wrote to a grower and wanted to order one and they said they wouldn't grow here.
About 10 years ago I went on a garden tour and someone here had several clematis...people were amazed. I finally found one (Will Goodwin) at a hardware store and bought it. I planted it, followed the instructions for pruning it and every year, I'd get MAYBE 4 blooms. I basically gave up on it for about 6 years and got the same results.
Then about 6 years ago I started seeing them in 5gal. pots at SOME nurseries...to the tune of $50.00 EACH !!! Those nurseries that did carry them only had Jackmanii, Nelly Moser and Wisley. Not wanting to risk losing a $50.00 plant, I passed. In late summer the same nursery had the same plants marked down to $10.00 so I bought a Jackmanii...only because it appeared it was one of the easiest to grow...and it DID grow...amazingly enough.
The following year I bought a Wisley...and it grew, too.
I was paranoid about the pruning aspect...doing it wrong, the wrong time, how far back, not doing the correct method for the correct group, etc. etc. and didn't want to push my luck.
NO ONE I knew grew them, so I asked Zuzu how she pruned hers and she basically said, "I don't prune any of mine. I don't pay attention to any of those pruning details because in our climate (hers and mine, essentially ) we don't need to since our winters aren't like the East coast...and mine do just fine".
I took her advice and have never pruned mine again...and they do very well. They get better each year with no fussing...just regular feeding and watering.
The nurseries here ARE carrying them now, but only in the spring...and the selction HAS increased. but it isn't vast by any means.
I'm looking forward to the ones I ordered from SimplyClems since they are reportedly good for this climate.
I don't understand why growers as well as garden books list some plants as not growing in certain zones. For a very long time, most of the nice daylilies were advertised as not good for zone 10...but I've ordered several and have gotten them in trades from people in other zones and they thrive here.
This just goes to show you that you can't really believe everything you read about certain plants not growing in your zone. I bought into that myth for too long !
