Pretty common in casual language. To express that something is possible but there might be a downside. She wants me to buy everything on the list and then pick up Johnny when school gets out. I don’t think it’s doable. Can you pick up Dad on the way over? It’s doable. (I can) It’s doable but we’ll be a little late then. (I can but ...)

Yes "doable" is commonly used. e.g. Yes, I do think it's doable to learn Japanese in 3 years. No, it's just not doable. We cannot issue a refund for a product you bought 1 year ago.