The main con is that you will not have work all year round, it was not the case when I started but Metlang has hired a lot of interpreters in recent years, furthermore you are subject to what the government is willing to allocate to different federal agencies which can vary every year, there are also other circumstances that have nothing to do with Metlang but work is becoming EXTEMELY SCARCE being 2021 and 2022 the worst so far, the trend does not seem to get any better which is forcing us to look for other jobs. This job is far from being stress free when it comes to your yearly income, there are gaps of employment since you're basically getting laid off when your assignment ends, there is no way to predict when you will be working again or even how long an assignment will last, these periods without work last several months. During those months you will depend on unemployment or another source of income. If you only depend on the work provided by Metlang, you will deplete your unemployment and hope for government extensions to be able to finish the year with some money in your pocket.
Not much room to advance, sure there is seniority preference, seniority will perhaps give you preference over who is called first to cover a job but that's it. You will be getting paid the same rate as someone who just started even if you have been doing this for over 20 years, there are absolutely no pay increases and they will pay whatever is the minimum stipulated in their contract with the government. If you want to advance in the company all that matters to them is how much you're willing to sacrifice in terms of $$ they will save, years ago these companies used to pay mileage when driving to assignments over 50 miles, there were other perks if you were working as a TDY, not anymore. I have heard of people driving over hour and a half every day just to get to the office for the duration of that assignment with no retribution other than their regular pay.
Some co-workers have been involved in serios car accidents not only due to driving long distances but also because they were driving late at night. Second shifts run from 4pm to midnight some 5pm to 1am., rain or shine, heavy snow, hurricane alert, pandemic when vaccines were not available, holydays or weekend, it doesn't matter you're supposed to be there on time. Double shifts (16 hours) and working the next day after a double is a likely scenario as well. College degrees, certificates, formal training and experience will get you nowhere in Metlang. On the other hand, sharing a rental car or lodging (when working a company sponsored TDY) or being willing to drive up to three hours a day back and forth from the office to your house without complaining may give you a slight "promotion" but even in that situation you will still depend on unemployment.