This actually touches on an interesting point. My notorious interview company is Google. I'm not familiar enough with the Hangouts Android client to tell you anything about it, but for the sake of argument let's unrealistically assume it's trivial. Imagine you have an organization where some engineers are good enough to work on the Hangouts Android client but not good enough to work on, say, the F1 distributed database.

Internal transfers become crapshoots. Is this person good enough to work on my new, highly complex project? Should I reinterview them to make sure they can hack it? Imagine what this does to the social and cultural stratification. "Oh, those Hangouts guys are nice but they're not that impressive, it's not like they're working on the machine learning or anything." It'd be a nightmare both practically for management and socially for the engineers.

Our philosophy is: all engineers should have the chops to quickly be able to ramp up on whatever project they like. This means rejecting a lot of people, but it also means that two engineers can look at one another across the hallway and immediately know they could swap teams and not miss more than a couple weeks of productivity. When framed this way, I think the interview process is a natural consequence.

Not sure what the culture is at other places, but I hope this is a useful data point.

But regarding your problem, you could solve that by having internal interviews.

And I really doubt that for any complex product people switching will only miss a few weeks of productivity.