If the chainplate is aligned properly and the seam area is thick enough, then polyurethane
will seal very well. It does not have to be 5200. Any waterproof polyurethane
sealant will do the job . Bostik, Sika, 3M, (i.a.) all have adequate products. I bet other materials will be just as adequate - I have seen polysulfides used and other stuff as well.
Where the seal fails is because there is too much movement in the plate (e.g. the plate not aligned, the plate too thin, the deck not supported properly, sogging, etc..) or else when the seal is not thick enough to allow for the movement there is. You can check out in the spec sheets
for each product how much movement is permitted without the bond being affected.
When we used our boat extensively, we would renew the seal every year. But I think you can renew it much less often if the boat is used less.