Why allowable for plate heat exchangers are very low as per API 662?

Can anybody through some light on above.

I would suggest this is because the nozzle connections don't typically have significant reinforcement available.

A larger reinforcement would make it more rigid and then these forces would be translated into the exchanger. By the nature of plate and frame exchangers this would then cause sealing issues from any translated deformation.

i.e. A stiff connection causes the plates to bend and then the plate gaskets won't seal!