I am with you in this but since they wanted only to add two more players i think they can does different choices...anyway better wait until the next will be reveal.
(Harry is a good flex supp imho... for that that i have some doubts but i hope him can make Fuel more strong...i want them over the Korean...boys in blues ftw)
This team is going to be actually unbeatable... I really think Custa is kinda underrated or like people forget about him in the best Zenyatta conversation and you could make an argument for his Mercy and Lucio being top 5 or 10 as well. His Ana is top tier as well but not as good as Chips.
As far as overlapping hero pools, I really do think that Custa is a better Zenyatta than Chips (its close) so I don't think they overlap that much.
Think about the option to run this lineup to pull Harryhook off of Lucio and sub in Custa and run triple DPS with Harry picking up the Soldier or running Chips on Mcree...
This lineup has infinite depth. The possibilities are literally endless. I can't think of a useful team comp that they couldn't find someone on their roster to play at the highest level...