I got a report that SyncEvolution does not handle CATEGORIES correctly
in ical 2.0 events when multiple categories are specified. It turned out
that Evolution or more likely, libical, import/export such events with
the comma separator escaped like this:
  CATEGORIES:Anniversary\,Holiday

RFC 2445 specifies CATEGORIES with a normal comma as separator and gives
this example:
  CATEGORIES:APPOINTMENT,EDUCATION

When importing such an event, Evolution stores it with two different
category properties and then only uses the later one in the GUI:
  CATEGORIES:APPOINTMENT
  CATEGORIES:EDUCATION

I could reproduce this with Evolution 2.10, but do not have the time to
investigate further.

It's probably obvious, but let me point out that any fix for this needs
to accept both \, and , as separator to remain backwards-compatible with
existing .ics calendar data files. It should use , when writing entries.