Pros
First, the clients are great reasons to go in: Planned Parenthood, Greenpeace, Black Visions Collective, RAICES, and others will make you feel good about what you're helping to fund and operate. The engineering team thinks hard about improving processes and quality of life. The technical challenges themselves are interesting: the stack is big and diverse, they use modern tools, and are open to the latest concepts. Small teams of 5 in engineering using agile means that you can tailor the experience to your preferences. The company's growing fast! 6-7 years ago it started growing from just political clients to non-profits and has been steadily gaining more and bigger clients. Add in a couple acquisitions and some VCs and the team is now sprawling. It moved into a new, spacious DC office 5 years ago and already outgrew it and moved into a bigger office last year. Others basics are solid: good benefits and PTO policies. Salaries aren't Google, but they're decent. Plenty of people stay for a long time, and for good reason: it's a good experience overall. Plus they're improving - pay, benefits, and PTO have all gotten better since I arrived.
Cons
Promotion process wasn't very transparent, make sure to ask about how you can grow over time. Some parts of the tech stack are older: VB, ASP.NET, and Angular 1. You'll have to use some of those older tools in addition to fun new stuff. There's also a decent amount of bureaucracy for engineers who want to take initiative.