> I don't recommend using minifiers which contain bugs

I recommend not using minifiers. Or compilers, for that matter. Or really any software. ;)

> or do unsafe transformations (a form of bug IMO)

I think it depends. In crass, unsafe transformations are only performed when you pass a special flag to the API or CLI. The constraints and effects are documented with the premise "If you're not doing X, Y, and Z, we'll squeeze out a few more bytes". X, Y, and Z are things like relying on browser bugs or only using deprecated features (e.g., using -webkit-border-radius without border-radius). Folks using CSS preprocessors (SCSS, stylus, LESS, etc.) oftentimes don't have CSS that could cause issues, and this option is safe enough for them (and provides a hefty benefit).

The problem with CSS is that you can guarantee many things as safe, but because of the flexibility of the language, it's impossible to minify "perfectly". Having knobs and levers for the developer to say "I'm not supporting IE9 or below, yes you're looking at ALL of my CSS, no I don't care about browsers older than three years..." gives much better results--especially on large projects--by trimming down the number of variables that could prevent different sorts of "optimizations".

Pretty much any transformation is unsafe under the right circumstances. That's why minifiers take a lot of config.

So you don't recommend using a minifier? In all seriousness, I don't think anyone is recommending using one with known bugs but there will likely always be some latent bugs that aren't known.