They're competitive but they're not drop-in replacements. Even office for Mac is not a drop-in replacement for office on Windows. It's pretty trivial to find significant differences that will be in use in any large organization.

Considering every job I’ve had in recent times has involved a switch between Google/Microsoft tools after being acquired, it’s about as drop in as anything gets in tech.

Of course no product will be an identical replica of the Microsoft tools, but both get the job done.

It depends. If you're writing documents and sending email, it probably not gonna be too tough. If you've got 100,000+ lines of Excel macros, you're gonna need a pretty significant migration.