I took the plunge- nothing exciting happened , the hack check is honestly pretty weak. I've transferred a lot of my old, "legal" battling Pokemon.
I've had only 4 Pokemon stopped. Two of them were Distrobro Golems that I guess were just a little too good being shiny and with awesome IVs, so oh well. The other two were a custom Zekrom and Reshirom that Richie made for me and Kloud for a present. So even though the Kyurems he made with them went through fine, those two are doomed to the void. Oh well.