I'm guessing that Apple's are better than average, since they have two versions (the built in HD on a time capsule doesn't make it appreciably different) and maintain them for long periods between upgrades.
Asus/Netgear/D-Link/etc follow the "If we don't release an 802.11ac router every week, we won't get enough press releases out!" model, and their firmware suffers as a result.
I'm not touching those unless I can wipe the stock firmware and replace it with Tomato or DD-WRT.