Rendering issues are OS issues too. For example, Safari's font rendering depends on the particulars of macOS/iOS's font-rendering framework. Safari on Windows would presumably use Windows' font-rendering framework, with drastically different results, due to the different frameworks supporting different subsets of e.g. ligature-pair width hinting or hyphenation, thereby causing text to wrap in different places, such that the layout at a given font size + viewport size on Windows wouldn't be predictive of the layout at the same font size + viewport size on macOS. (Which is not even to mention the fonts you'd have by default on Windows and
not by default on macOS, that would interfere with the font family cascade, and yet couldn't be suppressed from the cascade because
some macOS systems
do have them.)
Or, for that matter, there could be certain exposed -webkit CSS selectors that are really just features of some or another macOS rendering-layer framework, with no Windows equivalent — the same way that IE's ancient CSS "filters" were really just exposed DirectX features (e.g. filter: progid:DXImageTransform.Microsoft.blur). For these cases, testing on "Safari on Windows" would essentially just get you what testing in Chrome on Windows already gets you (unless Apple created an entire software renderer that brings in all of macOS's Quartz and all its subsidiary libraries as a big ol' polyfill.)
The point at which Safari is guaranteed to work like it does on macOS, is the point at which you're bringing in a polyfill consisting of 99% of macOS's frameworks. May as well bring in the last 1% as well, and call it a VM. (Ideally a thin-as-possible paravirtualized VM where the display driver is written to directly make DirectX calls, etc. — but VM client software is already pretty good at providing these sorts of drivers.)