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If you only read one sentence from this have it be this one: I 100% agree with Jayson, DO NOT DO IT. If you are going to join a startup, join a well funded, series A or B that has another designer. At least you'll have a team.
The following may be incorrect or wrong but based on my opinion these are very common. May of Jayson's points were 100% correct.
You will be on your own and as a young designer, not having a mentor will make your personal growth really slow down. Being surrounded by people who are better than you makes you better but it's important they are in a field you want to get better at. Designers need other designers around them to get the best growth in my opinion. I've been a solo designer for the last 5 years and feel like I've had to work harder to keep up with my peers.
Jayson's point about "respects your decisions less" is true even if you are a seasoned designer. Engineers/Developers/Coders, whatever flavor of the month title they want will think their ideas are better and fight with you frequently on your decisions.
UX/UI is so much more than graphic design. Many startups decision makers will want to be very active in the process to the point where they might start dictating down to you specific interactions they want because they saw it yesterday on Product Hunt and thought it was cool. You will spend the next 2-3 hours mocking up that concept and find it cant work and have to convince a decision maker on why their "idea" doesn't work with the UX/UI patterns are currently in place.
More on decision makers who are driving vision generally don't know what the vision or are still figuring it out. You will be doing a lot of work! The work will be done fast. A lot of work done fast usually equals less than great UX, not the best visual design, and not enough exploration or ideation. Your portfolio may not get filled with visually inspired and impressive work. I could easily get filled with mediocre work that never saw the light of day and in my case didn't leave me feeling truly proud.
To follow up with Jayson's last point "you'll learn business sides of things..." is my experience at every company I've worked at where I was a solo designer. At this point, I'm transitioning my career more towards front-end because I've become so frustrated with not being able to own design because the environments and team personalities. The places I've worked have not enabled me to get the final say on a visual style, UX, or implementation of a solution. Its become more design by committee and I've tried numerous times to change that. Due to time constraints, product uncertainty, etc. the workflow fall back to or become more based on what developers can build in short time frames than what is right for the end user. It has become more based on what the business development guy has been promising than what the end user needs.
That being said, if you like making power points, business cards and updating landing page designs for a startup, this might be good for you! You'll also most likely be under paid salary because your "lucky to work for a startup and the environment is cool!" The amount of equity they give you will to offset the under market rate be minimal at best.
Where the design community meets.
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