Hey @Aidas (KasValgyt) regarding customizations Twill is very open. It is your Laravel application and you can use any composer package. The whole frontend can be customized with Vue.js or vanilla JS, or even Alpine or Livewire if you're into that, you can create new form fields, etc... Regarding code coverage we have made immense progress in the past 2 years, and Twill 3 is now covered by a lot of end to end browser tests with Laravel Dusk. We even exposed test helpers for your own Twill modules tests.
Statamic and Twill share a lot of awesome features in regards to content editing, like live previews and image cropping, though I would qualify Twill as less opinionated and more open to a fully custom CMS organization.
Twill also doesn't use a "proprietary" format to store configuration, everything is controlled through code with Laravel provided features, allowing you to structure your code, use dependency injection, etc... as you see fit.
Regarding long term support, we are very active here and on GitHub, and are here for the long run. Twill was created in 2016, open sourced in 2018, and we have invested a lot since then, with the upcoming Twill 3 being our biggest release since launch. We have a lot of ideas in the roadmap and will continue maintaining and improving it for years to come. The community is growing every day and we can't wait to have thousands of people in this space.
Now, I don't think Statamic is hard to work with and it is a great product overall, so the decision is really up to you in the end, but know that we will always be there to answer your questions regarding customizations and will be excited to bring your ideas into the product if we feel like everyone can benefit from them.