Due to the hiring freeze, r/usajobs is full of applicants/appointees getting their job offers rescinded. VA
nurses,
social workers, and
MSAs.
Cybersecurity student and graduate programs.
Wage-and-hour investigators.
Tax examiners and
investigators.
These are positions that are only open to US citizens, and disproportionately go to veterans and recent STEM graduates. With the hiring freeze, the positions will be left vacant and the work undone unless and until contractors can take their place. Contractors don't have to be citizens unless the role requires a security clearance. If you wonder how well hiring foreign contractors works out, look at the Obamacare web site or VA health records.
It's hard to predict what knock-on effects this will have, but in the short run, government services are going to get worse and Americans will go unemployed. Then in 90 days they will have to re-announce all those jobs, recompete them, start vetting all over again, etc. Government efficiency is off to a great start.