I built a web crawling site ring.nz on polymer/web components. Looking at the marketing of the framework it was an easy way to get material design components but in reality the overuse of shadow Dom and shadow CSS meant it was really difficult to use the components or theme them. I'm pretty sure the whole polymer project is abandoned now... The site became too hard/bothersome for me to maintain under its tech debt but is still on my GitHub.

I also regret building some things in C# or Java a long time ago.

The biggest regret money wise is I built a crypto forecasting company that became somewhat profitable BitBank.nz but I built it without enough foresight for cloud costs, was running a forecaster constantly on compute engine as well as filling up a postgres database without offlining data, turned out harder to scale down a database than scale up the memory. Basically things like that and I never had time for it after having kids meant it long term costed more than it made even with paying customers so I shut it down.

Now I run https://text-generator.io still machine learning related but I run it very lean from two GPUs I own at home which is a bit easier