Biggest thing is that the ramp up time is very slow. You're not going to get a bombastic number of installs starting off, no matter how much marketing you purchase. It's a slow climb up and the best time to launch your app is yesterday. Plan on one to two years to see decent revenue, and then hopefully continue to grow it for the next few years after that.
Shopify Plus customers and traffic spikes are definitely a big concern, especially when it comes to your pricing model. Everytime I've come up with a "worst case" scenario it has happened so you need to always have built-in contingencies around that. Whether that means reaching out to the store to create a unique plan/pricing for them and possibly adding more value services. For example, one of our apps gets between 25k to 100k webhooks per hour and the spikes are unpredictable. Our app also has a queue system to process jobs in the background which can mean there are anywhere from a thousand tasks in queue, or 500k. (Shopify API is slow and limiting for big, heavy tasks).
If you are also an actual Shopify developer (theme or private app developer) there is a lot of opportunities for upsells depending on what your app does. It seems a lot of app developers in the App Store simply have apps out there to get leads for development (service) work.
Overall, developing on Shopify has been really good. There are some annoyances with their REST API vs. GraphQL API, their documentation, their support, etc... But generally it's been smooth sailing for the past few years. But we've had very, very slow growth and only in the past couple years have paid more attention on trying to maximize our business around Shopify apps.