I' ve been usin a cobra I' ve had for 10 years, and hadn' t had any problems with it.
I bought a truefire x-caliper that I paid $45.00 for, and after 6 months I had to send it back to the company for repairs. my cobra cost $24.00.
any good quality caliper type release will work fine on a loop, but now they make releases they say, are made especially for string loops. can' t comment on whether their any better than the regular caliper releases or not, but I doubt it.