Yes, there are many gradual paths to learning Unity. I would highly recommend following the official introductory tutorial series. This covers core topics like Components, Transforms, Prefabs, Game Loop etc. After this it gets a bit less clear how to proceed. If you're already a competent programmer I'd go straight to just prototyping ideas and reading the documentation in depth. I would especially recommend reading the best practices guide.

Regarding the development environment, yes it's very integrated and isolated. For me this was a bit of a culture shock as I generally prefer working with text rather than IDEs, but I've gotten fairly well acclimatised to it now.

The closest analogy would be the Flash IDE. Art (3D, 2D, images) is made outside. So is any coding. It integrates nicely with visual studio and Blender (and Maya and Photoshop and the like) so the process is fairly seamless. For example, debugging works.

Yet it produces native apps as well as WebGL.

An infinity of tutorials exist in as many qualities so learning resources of every level are hyper abundant from gradual to accelerated. A passable 2D app is probably possible in a few days starting from zero knowledge. But a impressive 3D game will take mastery of quite a range of its abilities and probably require a team.

Since it is not dependent on a proprietary player, it probably has more staying power than Macromedia/Flash but it does have competition from Unreal which I imagine is a viable alternative.

I built and published this 2D app in Unity with only prior web experience in a weekend: https://youtu.be/NTc6pf6OrI4

Haven’t touched Unity since... even though I’d love to make games. I’m partly afraid of future maintainability of my projects, but there is a potential future where 3D engines converge on a standard... maybe never.