From the videos I've seen about Stage Manager (SM), it seems like a nice feature for those using a single small screen (laptop or iPad).
But I'm not sure how well it would work for me, because I use three monitors in an extended desktop configuration, and I'm not very disciplined about where I throw up new windows.
With Mission Control, with a single action, I can see all the windows in a given app, or all the windows total, across all my monitors at once. By contrast, SM doesn't currently have the capability to set up a single SM that works across all monitors. If it did, I might anchor SM to the LHS of my L monitor. Instead, IIUC, when you activate SM, you get a separate SM for each monitor. Thus if you don't know what montitor the window you want is on, you'd might have to check all three SM's before you found it.