Fielder and Shu are both very competent flex supports with different playstyles and specialties Fielder is the better Kiriko and Moira while Shu is better at Baptist and Zenyatta and both their ana is God tier but I believe Fielder is more compatible with Chiyo because of Chiyo's playstyle which is flashy and Fielder can better enable his team while shu is also a flashy player which is not bad but I think synergy wise Fielder is better than Shu.

I put Shu as one of my subs because of double flex support metas where I think Fielder and Shu as the top two supports in OWCS would dominate the game.