1. Extremely high bureaucracy, very poor facilities management. Expect a terrible shared office in a temporary building with asbestos. Dont expect to be reimbursed for your travel expenses for like 3 months, and you're very likely going to have to travel for conferences (i.e. fly to DC to make your sponsors happy).
2. The old joke around LLNL was, "hey, you know how many people work here? About half." Half the people you work with will be the smartest most productive (and friendly!) people you've ever met, the other half are also smart but have realized that their productivity has no bearing on their advancement and so have decided to not give a fuck. Since hiring is such a nightmare, its very hard to get fired. From a program managers perspective, they'd rather keep people around who massively underperform then fire them since getting a replacement could take years. The upside is that if you want to get paid to do nothing, this is an amazing place to work!
3. You dont need a clearance to get hired, although it helps, you get to do the incredibly invasive FBI-agents-knocking-on-your-moms-door clearance process after starting, and then again every 5 years for the rest of your time there. The upside is that you get to make really fun Qanon jokes with all your coworkers. Be ready for random drug tests as this is a Federal facility and Cali's pot laws have no affect. Eat a gummy at a party? kiss your career goodbye.
4. If you're cool working on nuclear weapons, you're set for life. If you dont want to operate the gas chamber at Auschwitz (how I see nuke people), then your funding will perpetually be in danger, and you will likely spend more time chasing grants than writing code. (Nearly) Everyone you work with will be totally OK working on a tool with the express purpose of killing 100MM people. I had friends/co-workers who couldnt talk to their spouces about what they did during the day. The nuke people are also very enthusiastically pro-america in a creepy way that always set my teeth on edge. All diversity/inclusivity programs here forcibly killed by the GOP, which leads me to:
5. Politics affects everything. Government disfunction is annoying enough, but if there's ever a fight over the federal budget or a government lockdown expect it shutdown your work as well. If you work on something politically sensitive (hello climate program!) expect your funding to be on the chopping block. Real shit, when Trump came into office my entire program changed its name to exchange "climate" with "earth system science" to try to run under the radar.
6. (LLNL Specific) Livermore is very expensive (about the same as the rest of the Bay, but still pretty expensive by anyones measure) with none of the things that make the Bay nice. No BART stop and massive traffic means more than 2 hour round trip to Berkeley and back. Your Bay friends are not going to come out to visit, and in terms of travel time Sacramento is closer than SF. Unlike the rest of the Bay, its temperature is not regulated by the ocean so it regularly hits 115 degrees for weeks on end during the summer.
All that said, I still (mostly) enjoyed working there. The reason I left is because during covid I moved to SoCal and after the climate program got its funding reduced the HPC/Nuke people who wanted to hire me onto their team wanted me to come back into the office (there are certain terminals for accessing classified material that are fixed in place and cant move).