Yes, you can use it on a Mac or a PC, but that is tethered, you need to give permission from your phone and snap a code with your camera. I mean completely independent access from another device whether it is a computer, tablet or another smartphone, just like you can with Messenger and other apps. Right now if you actually access WhatsApp from another phone you lose access from the current one, I learned it the hard way and lost a bunch of info.
Regarding WeChat, I agree it is clunky but you have a larger feature set, probably Telegram is a better example but my point is there is a lot of room to make WhatsApp better and a look at the competition is a good start.