Nissin’s flashes are very similar in function to canon’s and in some ways better. The new ones have a USB port so firmware can be updated if canon change their flash software and the risk of problems is really quite small since nissin license the software from canon. In fact, both metz and nissin have been making flashes for far longer than canon has.
As for cheapo flashes like the yongnous, they work perfectly fine. They are well built and have decent specs but cost very little. If you aren’t a professional or have a big income yongnous would be the way to go I think. You can get yourself four manual flashes and a radio transmitter for less money than a single 600 EX. Nowhere near as convenient as the canon offering though. You can also get their flashes with TTL support and TTL transceivers so things will be properly exposed without too much hassle and it lets you adjust ratios between flash groups through the camera. There is little doubt in my mind that the canon system is better but it is also far, far more costly but can be relied on fully.