The answer to your question requires an answer to another question:
What do you mean by "harder to get into retreating blade stall"?
If you mean, "Is it harder to put the retreating blade into stall", then the answer IMHO depends more on blade design than the system that allows the blade to flap. To partly address Mr. Lappos' comments, in a semirigid system, the designers' aim is a design that will balance out the dissymetry of lift in the disk through the up-flap of the advancing blade and the down-flap of the retreating blade. In a fully-articulated system each blade finds its own "balance". Regardless of the flapping system, the retreating blade will eventually stall given sufficient forward airspeed. It's the design of the blade itself with features such as twist that increase the airspeeds at which a retreating blade will stall.
If you mean, "Is it harder to get the aircraft to depart controlled flight due to retreating blade stall", then the answer is most probably no. A semirigid system will depart controlled flight sooner than a fully-articulated one. But this is, again, not principally due to the flapping system.
Typically, fully-articulated rotor systems have more than two rotor blades, but only one rotor blade is in a stall condition at any given time. In the airframe I presently fly, there are four blades so when one is in stall, three are not. Retreating blade stall manifests itself simply as a 1/4 vibration that increases in intensity as you increase the stall. So you usually really have to push it to get the aircraft to depart controlled flight.
Semirigid systems, OTOH, have only two blades. When one is in stall, only one other blade is still producing lift, and it's all on one side. So departure from controlled flight happens fairly rapidly and sometimes violently. I can attest to that from personal experience...
