Can switching between two unsynchronized sources cause problems for an average office building?
ATS are designed to be break-before make, to avoid the case where the genset is connected to the utility (a very bad thing for many reasons, and generally not allowed.)
When the ATS transfers, it would be as if the supply were interrupted briefly. You mention 50ms dropout time. This is several cycles. There's no guarantee that the phase alignment of the transfer between the utility and genset will be smooth, unless the ATS and genset were designed to do that.
What's in an 'average' office building nowadays that will care? It will include some critical IT stuff like servers, network and telephone gear, not to mention life safety and other building management.
On the IT front, PSUs in computers will not hold up for 50ms. ATX (PC) power for example will only hold up for 17ms, basically one missing AC cycle. Good design will have mission-critical IT gear on a UPS, regardless of having onsite generation capability.
For other mechanical and building infrastructure gear, check with the manufacturers. They should be able to restart correctly after a dropout.