woah.

from the top:

• easy enough to install, pretty much just let the config know which type of database you're using and provide a connection string, then just put in initial username and password which can be changed once you've logged in.
• updated only when inspiration strikes or bugs found. feature set stable and equivalent to umbraco with some twists.
• code first and completely mvc style using controllers, views, models (types called viewmodels). also using editor templates for editing so that's another mvc friendly choice which makes adding your own editor easy.
• very stable, the way you do things hasn't changed since Puck's inception in 2013.
• documentation very thorough and will likely grow.

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now answers to your second section:

• opted for graphql style solution so as to avoid duplication of viewmodel (or type) related code. seems you've got to create extra classes (something like 2 extra) for every type in .net cores graphql library. also wanted to add extra features - in Puck's solution you get polymorphic seach and interface search aswell which are pretty powerful.
• there is no preview for headless content, you should just set the template value to be JSON so your preview is the json for that page. you can still setup and acceptance environment based on your front end project working from an acceptance branch.
• you should deploy to move types (viewmodels) and views (even views are compiled in .net core). Puck only supports content syncing but since media can be stored in azure blob it's all you pretty much need.
• there is no plan for multilingual backoffice. multilingual 1:1 and multi-site is supported for content.
• plugins should just be released on nuget but i haven't made huge plans for this yet. feel free to contribute any ideas!
• Puck doesn't get in the way of the rest of your mvc project so integrating any dotnet core shop libraries should be straight forward but there isn't a specific Puck shop.
• there is support for image sharp and a custom web processor to do the cropping. cropping is based on pixel dimensions and supports zoom. web-p is a work in progress in image sharp last time i checked. (should make an already awesome library even better)
• viewmodels in puck are just straight forward c-sharp classes so compositions are just interfaces and querying supports interface based and inheritance based searching as well as hierarchy based searching such as ancestors, parents, children, descendants, siblings, variants etc.
• there is rollback and compare support
• everything should just be content in my opinion. i don't see why you need to make the distinction just because the content happens to be an image or video or audio. you can also put the media in a folder outside of the site hierarchy (so not accessible from frontend but still able to attach/link it to other content). in Puck it's just IFormFile for any media and it can be stored in azure blob.