Quality of employees varies wildly. Sometimes you're working with ex military that take security seriously and have great problem solving skills other times you're working with college kids who could care less, have no training beyond the minimum and some places you might work with people who barely speak English.
Benefits should be way better for the size of the organization. Medical is high deductible, don't expect a raise.
All training costs are your responsibility but sometimes there is a corporate discount
You can be removed, reassigned, or have your schedule changed at any time, for any reason with no recourse. I found out about a reassignment from a coworker not my superior.
Pay is determined by contact so you could have two employees doing similar work at different sites at significantly different rates.
There are no shift differentials so you'll be making the same rate regardless if you work morning, overnight, or weekends.
You're just a number. There is no loyalty or appreciation from management. I've had supervisors promise the world to get what they want then just hang you out to dry.
Some sites can be dangerous. I've recommended armed guards or more personnel be added to a site but all you will get is: we only provide what the client requests in the contract.