I transferred my Xbox characters to my PS4 seamlessly. Cross-Save is a gem. My problem is this: I already owned base D2 on PS4 and I obviously still had to buy Forsaken to access the content I already had on Xbox. However, I was hoping, with my (second) purchase of Forsaken that I would receive another character boost, but I did not. I don't want to purchase a $30 character boost after I JUST bought the D2 Forsaken Complete Collection because it was the only bundled version of Forsaken (+ Annual Pass) on the PS4 Store. I feel as if it is only fair that I should receive another character boost for having technically bought the game a second time.
The character boost is part of the license purchase, and as such is tied to the account and that account's save data. Your purchase of Forsaken on the PS4 gives you a freebie that allows you to boost a PS4 character. Your purchase of Forsaken on the Xbox One gives you a freebie that allows you to boost an Xbox One character. As PyroAJM mentioned, if you have cross-save enabled for your Xbox One characters and have those characters available everywhere, and you purchased Forsaken on the PS4, then your currently-stored PS4 characters have a freebie boost waiting should you ever decide to disable cross-save and make them usable again. Your purchase of Forsaken on the PS4 will not grant you a second freebie boost for your cross-saved Xbox One characters.