The hard spot created by a POINT load of a stringer against the hull is prevented by bedding the stringer in PB or PL. The stringer has a small profile along it's narrow edge. The bedding creates a wider cross section that spreads any loading across a much larger cross section then 'just' the narrow plywood edge. That would be the hard spot....
Some manufacturers raise the stringer off the hull & while suspended, tab the stringer to the hull w/ fiberglass alone, no bedding......... Of all the stringers I've seen posted to Iboats, the large majority of stringers delaminated (glass or tabbing broken/torn free) from the hull were installed w/out bedding. <<<--- Not proof positive that bedding is required, but enough for me......
PL Premium dries in the middle ground somewhere between fairly flexible & fairly hard. PB dries hard, and although if you search thru my posts, I've often supported PL's use for bedding primarily for it's somewhat flexible bond, a great positive in my mind, it does have drawbacks. And you need to be aware that it off gases for 72+hrs, so further work is delayed. As it cures it bubbles when applied heavily or in thick beads (like a fillet), sometimes it bubbles a great deal.
I've not seen 3M 5200 suggested as an alternative bedding compound, but I do think it dries to a somewhat sandable surface, but it too takes quite some time to cure, maybe even longer then PL.......... May not bubble as it cures..... It's expensive... I'm not sure it would make my list of favorites for bedding......
Although fiberglass & resin once cured might have some 'give' or flexibility to it, I don't think that the Shelly's flexible PL will remain that flexible once glassed over......
Since you're working on a fiberglass boat, and will be using polyester resin & fiberglass, I'd suggest that you use PB to bed the stringers, make your fillets & to bond the transom to the fiberglass transom skin. You'll be using a bunch of poly resin & etc to make fillets and for all the glass work, and buying the materials in bulk anyway.
Just my $0.015
If you want to give the Shelly's PL a go, I'm sure the rest of the Aussie/NZ iboaters will benefit from the results you post, for good or for the bad..........
Hope some of that ^^^ helps.........
