I've done exactly that, worked as a Red Team Lead, and the success rate is pretty disturbing. That, and vishing - calling people from the company you find on Linkedin from a spoofed number of their IT that they fucked something up and need to download and run this .exe to fix it before The Audit that's currently happening notices it.
Even if we do internal infrastructure tests where they let you in, switch AVs to "detect mode" instead of "block mode" and the goal is to find as many unpatched systems/vulnerabilities as you can (instead of, well, testing the AV solution), what we usually do is run a password spray for all domain accounts with a combinations (you can try like 3 to not lock the accounts) of "" we every single time got at least few accounts.
Fortunately this kind of tests are getting more popular, and passwords such as this should've definitely been caught in some kind of security test. But it is also pretty depressing, when you repeat the same test next year, and 80% of the passwords are still the same, and vulnerabilities are still not patched.
It depends on how well segmented is their network, but all you might need for that is a Raspbery PI with ethernet and GSM.
I've done some engagements where we sent someone into the company to get in as an air conditioning tech, and when they got in he planted that device between a printer and the network. It was set up to forward all traffic, but also allowed us to connect through GSM and get into the network.
It takes like a few seconds to plant it.
Or if it's really bad, then you might be able to reach it from the WiFi.