The USCG 100 ton ticket is a joke internationally. It can be gotten by anyone who fakes sea time and takes a course on how to pass the test; there is rarely a verification process. It is not valid in any other country and does not equal a certificate of competency, it is only a permit to operate vessels certified by the USCG.
I know nothing about the RYA system other than it is recognized in the industry as the standard for capable and qualified mariners.
On the other hand, my Commonwealth Pacific Islands Certificate of Competency as a Mate was a 25 hour examination after I operated as harbor pilot, docking/undocking all merchant vessels for a month, not some silly multiple choice exam.
I have worked under several unlimited master's certificates, the Commonwealth certificate and a USCG 200 ton Master's License from the USCG and can unequivocally state that the USCG paper is worthless as a testament to a person's knowledge and abilities.
Well, I guess, I've been told. Thanks for clearing that up.