Since the front and rear legs are trapped in there respective lips ( for lack of a better term) the joint which appears to be weak is actually very strong.
Certainly, like every design, there is a compromise between appearance and strength. I build my stuff to take what I consider normal use, not abuse.
On a 24" seat height chair or stool, rocking back isn't an option as your feet aren't on the floor.
I feel in most hardwoods this will stand up long term. No way I'd use pine for instance. Also, if you are a larger person it might be wise to add some supports. But for the customers I'm making these for ( well under 200 lbs even the father) not going to be an issue.
I'm going to design a bar height (30" seat height for a 42" counter) based on these and there will definitely be some stretchers added between the legs.
My customers know I stand behind my product 100% and if anything happens down the road I'm good to repair or replace.
I've done repairs on furniture my father built 35 years ago, and haven't charged the people. How many furniture stores will do that or even have been around for that long?
AJC