No material cons to speak of, though several of the "pros" could be seen as double-edged by some.
If you are looking to skate by with low effort, or simply disappear into your work without engaging with your team, this is not the agency for you. If your talents are overwhelmingly concentrated in one area of performance but sub-competent in others, you will likely not last long. Single Grain loves to hire "T-shaped" marketers, but the top of the 'T' always contains communication and personal accountability skills as much as it does technical skills.
I would emphasize that it is very difficult to get hired at Single Grain. It is empirically easier to be hired as CEO of Single Grain than as an intern. Strong resumes, relevant in-field experience, and devastating intelligence will only get you so far: the ground beneath your feet must quake from the weight of your passion for marketing. You must bleed advertising, and skillfully communicate it so. Still, this too is sometimes not enough, and I have witnessed several qualifiable hires be turned away due to a perceived failure of a final "cultural" test of worthiness to join the team.
Culturally speaking, the company has made substantial improvement in aligning its vision to something compatible for all team members, but there is still progress to be made here. A "growth hack" and "guru" mentality does inflect much of the culture, though it's arguably an indelible part of the DNA of the company and even more arguably part of the new normal for small agencies. There is a valid question over which business objectives are served by which teams and departments, and while the answer would likely mention the phrase "scrappy startup" in it, some may appreciate more clarity on where Single Grain ends and the founder's personal satellite projects begin.
Lastly, the work itself can be tremendously challenging, even as far as agencies go. Some days are a breeze, some days are war. Every employee should expect to mark individual successes as well as defeats - such is the nature of the beast - but know that you will need to understand your own limits at the same time that you expand them. Take advantage of the generous PTO offering and prevent burnout before it strikes!