SXB
Interesting point.
I am a UK citizen living in Malta and hold a Maltese ID card.
It has the words "Karta Ta' L-Identita" (Identity Card) clearly printed on the front and is issued by the Commissioner for Police per the ID cards issued to Maltese citizens.
The only differentiator I can find is that the number contains the letter A as a suffix, which designates that it is an ID card issued to an alien.
When I use it to travel around Europe and Scandinavia, I get some interesting reactions from immigration officers...
France (many uses) - no problem, its an ID card issued by an EU state, within its validity period and the photo matches you
Finland (on 1 of 2 uses) - never seen one of those before, it looks fine, but do you mind if I show it to my colleagues so that they can recognise one next time? - 5 minute wait whilst card is passed around several booths and then returned with thanks
Germany - no problem, per France
Belgium - no problem per France
Switzerland - immediate reaction is not valid but unsure, I offer to show passport instead, but officer calls manager to clarify as it is important to him to know. Manager says its fine since May 2004 (date of EU accession)
UK - twice - it's a residents card, not valid as a travel document. When I point out that it says identity card on the front and produce my Maltese residence permit - which is in the form of a certificate, not card and is in English, one UK officers said that she does not speak Maltese and cannot understand the words on the front of the card. I could have been a prat and insisted that they called a translator, but life is too short. The other just refused to accept that a foreign government could issue an ID card to a UK citizen.
I now use my UK passport to avoid the hassle, although it is a shame to have to carry a relatively bulky and fragile document instead of a document the same size as a credit card.
The 2nd UK authorities may be correct in their view (I have not read the EU law at a small print level, if indeed it exists at that fine a level, in whether a 2nd EU state can validly issue an ID card to the citizen of another state), but it is interesting that they are the only border control authority who have taken such a strong view.