Thanks for reaching out to the community @SimonKubica. This is a sign, I believe, so many vendors have been waiting for from Atlassian. I am afraid I guess many answers will go beyond your question because there is a lot more broken than just the platform or API experience.
I agree with everything that has been said by others. In addition to that, I would like to highlight an issue with Atlassian Connect frameworks.
Atlassian officially offers two Connect implementations and I would like to focus in particular on atlassian-connect-express. The state of this project seems questionable to me to say the least. There have been no real Atlassian contributions to this repo for more than half a year, the repo has 10 open pull requests, some of them as old as 2017, and the project’s Jira board seems to have been frozen since mid 2019. Changes that have been merged since last summer are all vendor contributions (some of them security patches to comply with Atlassian’s security guidelines!).
How can such a big company that takes 25% of vendor-generated revenue to (hopefully) fund development of such essential platform tools, neglect a project that much? I understand that it’s all about Forge now for Atlassian but they seem to forget that there are thousands of vendors relying on the existing tools. How can a project so essential to a platform be abandoned like this and how can Atlassian simply leave it up to the community to maintain a project (without giving them actual control over it or involving them)?
I think you can see where I am going with this and I believe it reflects the same feedback many others already gave from yet another angle. Atlassian seems to be more and more focused on itself and detached from the ecosystem.