Unusual’s Startup Field Guide says: We’ve found startups to be less optimal design partners...While they have extreme urgency and can certainly move quickly, they lack the stability and consistency that a [larger] company has. In addition, they often have small budgets and a high propensity to “DIY.” They are often consumed with their own challenges and feedback so you can easily get lost in the shuffle.
Don't forget: extremely fast feedback (ex. via slack connection), low expectations, fast growth in usage, short sales cycles, and more