I mostly play solo unranked. I'm not sure if it's because of the BP introduced separate ranked mode or if I really have a rather high hidden MMR (5.7k ranked but hardly ever play this and I wanna argue I improved a decent bunch. since then) but fuck it if I'm not averaging 8-10mins wait time currently.
I think the less competitive nature of unranked can be used well to improve yourself. In areas where you aren't quite as sufficient as you want to be, it can be hard to actually get there when the game demands your very best every second. If you for instance want to get extremely good at a specific hero, you need to get accustomed to it first and that's easier to do in a more relaxed round. You still need to play with the intend on actively improving, of course. If you play Tiny and laugh how easy it is to get a good avalanche/toss combo out instead of actually trying your best to make the jump in difficult situations, too, or be more experimental in general, then that won't help you improve.
edit:
That said, the MMR disparity between players can be too stupid at times in unranked. Again, might be because most people at my MMR level prefer playing ranked but if you get legit 4k players paired with/against 5.7-6k ones, that's just gonna suck. Average team mmr doesn't help there too much.