I have no problem with the technical underpinnings. Or rather, I think (almost) every programming language today is kind of lousy, and the choice of Vala doesn't scare me off any more than anything else would have. No platform in the history of computing has made a great choice for its primary language, so I can't hold that against them.
When a platform gets the user experience right, developers will figure it out. Developers complain endlessly about JavaScript, but they want to target the web, so they made it work. Or Objective-C, and the Mac. Or C++, and Windows. We like complaining, but a non-ideal programming language really isn't a blocker.
The issue I have is that I don't see a great user experience here. This new brand of "consistency" just looks confusing to me. Every possible UI control -- from window title bars to tabs to buttons to lists to labels to text fields -- is a plain gray rectangle, with the same color and border. Am I supposed to figure out everything from context? I don't have much confidence that I'll be able to learn a new app, much less write one that other people will be able to figure out.
The metaphors, where I can find them, make little sense. I don't know why I'd want to "flick through" a grid, to select from a list. How is this better than scrolling through a list? They seem to be blindly copying parts of Apple's systems, without understanding why or when Apple made those design decisions. There's so many obvious rip-offs of macOS here, it's sometimes hard to tell if it's meant to be a parody.
And for some crazy reason I just can't get over the lone word "Applications" sitting in the corner of the screen -- the only word anywhere. It looks like a mistake. At best, it's clumsy. My steering wheel doesn't need "Steering" printed on it.