I'm very surprised that some of our more experienced boat repair people haven't commented on your use of 5200.
5200 is an amazingly strong adhesive. If there are two parts sealed with 5200 on a deck and for any reason, even expansion and contraction due to two different materials, something has to give. The adhesive will not give, it is too strong, so another crack forms creating a worse leak than before.
Ok, I'll bite. 5200 had nothing to do with Glenn's problems. I've used over 3 CASES of 5200 in my day. Most of it was used below the waterline to keep water out of the seams of the hull of my 31' leaking lapstrake boat. The rest was used (since I bought it by the case) to bed hardware, seal hatches, fill in rotten spots in the deck, and spread on my toast.
5200 has all sorts of legends surrounding it, and few are deserved. My favorite is the tripe that was reproduced in a "West Advisor" article in a West Marine magazine about 5200 holding a keel on a sailboat. Tried that, twice, it's bollocks! (photos available on request). I can tell you that a bead of 5200 has the tensile and compression strength of a gummy-worm.
It's adhesion may be good, but it's still a rubbery flexible caulk. It will expand and contract, flex and all that. My boat's planks flexed quite a bit and the rubbery stuff stuck to the hull, flexed along with it, and didn't tear any wood out.
Most of the legend that separates 5200 from all the other polyurethanes is just ill-deserved folklore.