I'm a big fan. Variable fonts aren't just neat -- they're a godsend for graphic design when making things like posters, packaging, etc. To get the font weight
just right, to get the letter widths
just right. When dealing with a font that comes in just 2 weights or even 5, it's so common to want something exactly
in between. Or when you want the typeface to be condensed, but the condensed version is way
too condensed -- you want "halfway condensed" which is almost never a thing.
And I don't think most variable fonts require any kind of Turing-complete programming language? They're basically just interpolation. And interpolation is not that much added complexity for the rendering engine.
I agree with you that the kind of Turing-complete calculations that fonts these days are able to do, make me feel uneasy as well. But for me, variable fonts don't fall into that category. I look forward to the day when variable fonts are the norm for typefaces that come in different weights and widths.