I’ve found that ch and ex units are heavily influenced by latin characters. They just give weird results with non-latin characters leading to magic numbers. But the concept is really solid: use them if you want the spacing relative to text.
It's often easy to spot Chinese goods because English writing on the stickers or manuals is in that horribly ugly latin serif font with bad kerning, which almost certainly looks fine if you don't read English all day.
(The difference between rem and em is "root", ie, only using the top-most font size and sometimes the font-size before things like zoom and other similar adjustments. If a designer complains about the `r`s you removed, sometimes just showing them the difference at multiple zoom levels is convincing enough.)
(The difference between em and ch is em is wider. If a designer is always giving you weird decimals like 0.79em or 1.24em that may be an indicator that they themselves would be happier working and thinking in ch. em is the width of the capital M which is often the widest letter in most latin sections of fonts. ch is the width of the number 0 which is often the most "typical" width in the latin sections of fonts.)
(ETA: Both of which relate to the comment above complaining that both ch/em and others fall apart when dealing with non-latin scripts, because they are established as latin-centric.)