The only reason you'd use one of the multiplatform engines like Unity or Unreal would be if you didn't have a multiplatform engine already, or particularly wanted a decent 3D pipeline and didn't want to take the time to write your own. I don't see Unity, or similar, as the be all and end all of multiplatform because with each version it seems to break some portion of the code you were relying on previously. So I would expect it would be a pain to recompile the game in 5 years time if I want to make some minor changes.

AGS and Wintermute are both very mature for 2D point and click engines, but lack the deep multiplatform support, especially for touch devices. Visionaire was nice, but I remember there was some platform drawbacks. The big problem to be solved, I find, is the sprite-system, especially sprite packing, for mobile devices, if you want to support high resolutions, while also being able to run on lower resolution devices. And because of Android fragmentation, if you want to support that platform you have to consider that resolution soup from the beginning, at design time.

I still think Lua is pretty good, but a hassle if you have to compile it for multiple platforms, and you have to be carful how it performs on mobile, if it's consistent between desktop and mobile. I find C# really good as a scripting and programming language, but multiplatform is not straight forward, though Unity seems to have solved this, but I don't think it's straight forward. C++ is obviously king for multiplatform.

Honestly, no-one has really solved it definitively for 2D point and click and multiplatform (especially for touch). And there's not a lucrative market for it so your best course of action, which I'm sure you've no-doubt surmised, is to roll your own solution.

DoubleFine mentioned they were going to release their engine, built on Moai, when it released, but I think they did so much work on it that it's too closely tied with their proprietary tools to be any good to anyone.