Pros
So many, especially with the benefit of time. You work on projects of different sizes and in different fields/industries. You will have the opportunity to lay the foundation for what will eventually be a large and substantial project--very much not the typical experience at larger companies. Your title is software engineer and you actually spend most of your time developing! Not so easy to find that these days. The owners/founders of the company are also typically the project leads, and I had a great time working with them. Not too much management and not too little. Laid back yet results-focused Good intellectual climate. Weekly 'discos' (discussions), i.e. talks, on software and EE topics. The owners have a sincere interest in employee well-being and maintaining work-life balance. This is non-trivial. At the yearly retreat many of us asked for better benefits. We (all) got them. The partners could have indefinitely tabled that request or provided some nominal improvement but in fact they were increased very substantially. No drama or politics I experienced. Colleagues are sharp, teams are usually 2-3 people incl. project lead. Remarkable level of transparency in the operation of the company. Company seems like it's on a nice upward trajectory, and is definitely moving with current tech, Check out their news page and do a google search on news items (can't link here).
Kontras
Pay is less than what you can get a larger companies. At least when I left, it was purely a consulting company, meaning its only revenue stream is billable hours. This constrains a bit what the owners can do in terms of compensation. Clients set the terms of how you work for them and while sometimes that's great, sometimes it's less so like when this one guy was appointed my 'gatekeeper' and was telling me how to code even though he had no particular expertise in what I was doing (gui design).