If you've got the money I'd recommend the Honda, or a Stihl, but if the Ryobi can take a metal blade and 4mm cord it might just manage. Much depends on the design of the head - Honda and Stihl are both excellent, but the McCulloch I had was rubbish. Sorry, but I've not used a Ryobi. Incidentally, I've found the 4-stroke honda to be much more fuel efficient than the 2-stroke Stihl.
Bramble and blackthorn scrub is no problem with a metal blade and I wouldn't use anything less than the black square 4mm cord if you don't want it breaking all the time.