From internal bug 18782818:

@raphlinus:

[...] I researched the shape of "χ" in a number of other contemporary Greek fonts, and see that the
version without descenders is quite common. For example, it appears in the Ubuntu font, which was
developed by a highly respected type foundry. In addition, in the discussion of
http://typophile.com/node/29711 it is stated, "chi's and eta's without descenders are something that
more and more appears in contemporary Greek typefaces (a big percentage of typefaces designed
and used inside Greece)".

It's a bit controversial. In http://blog.gmane.org/gmane.comp.fonts.dejavu/month=20061201 Ben
Laenen states: "That is one of the concerns I'm having, and also that the Greek should be useable for
ancient Greek as well. Therefor I'm still not too keen on the eta and chi without descenders." [...]

@roozbehp:

This appears to be a deliberate change by Christian in Roboto 2 (Roboto 1 has a different chi than x).

I personally don't like it: Roboto is now a default font everywhere, and Greek is not limited to modern
Greek (there are lots of people using chi on the web for basic math, and they are now seeing their x's
and their chis the same way [...]). Basically, I think we want to be more conservative
with a system font.

I also compared with the Windows 10 sans system fonts. Arial, Calibri, Segoe UI, and Tahoma, all
differentiate their chis and their x's, I believe for similar reasons.

[...] I think it needs to be fixed in Roboto 3.

@christianrobertson:

Valid feedback. We should do more research on this question. Generally I
agree with the point that we should choose the more conservative route for
the system font, though in this case the answer isn't 100% clear. The math
point is interesting since there are other characters where the expected
math version of the symbol doesn't work well for running text, or has
diverged from the common usage. In some cases there are multiple code
points, but as far as I know it's not the case for chi.

Feedback from native Greek Googler:

Just wanted to add in this discussion the fact that we (native Greeks) use
'x' as a multiplication symbol, which is another reason that if things stay
as is there will be some major complications. ''x" is never used in Greek
either as the English x character or the multiplication symbol.

In schools, we are taught to write chi as chi and ita as 'η' but without
the longtail. 'η' is actually considered an older form, no one is writing
it like that at the moment (I should perhaps file another bug for this),
but that seems to be more innocent than using a character from another
language (x instead of chi). [...]