I don’t know what is the specifics as to why, but when I see a poster or something, I can instinctively feel if it is made with Canvas. I don’t know if it is the font, colours, how generic the templates are, or if it is the stock images, but I instantly feel a repulsion towards said graphic design. It doesn’t matter what style or vibe it’s going for, but I instantly hate it.
I think I’d have to see examples to give you a real answer, but I think one thing you see a lot with Canva is pieces built by skilled designers slapped together by someone who might not even be artistic at all.
I’m a designer myself, and I’ve had many times that clients have tried to do something in Canva, only to get stumped on why they can’t get it to look good. It’s usually stuff like bad hierarchy (the relative prominence of different elements to each other) and bad composition. You can have the most beautiful stock art pieces in the world, but they won’t fix your composition.