Hi Marko,
IMO, mkgmap should not silently ignore anything. We should generate a warning too.
BTW, one more case that I do not know if you are handling yet: Issue a warning if the result would not be any different if the turn restriction did not exist. This is not only about oneway direction. For example, if there is only_straight_on on a road with no junction, a warning should be issued.
Sure, whenever a restriction is ignored there should be a reasonable explanation.
BTW, I accidentally loaded a relation somewhere in Russia and downloaded the surroundings. There were lots of no_u_turn on every crossing of a major road. That kind of micro-mapping does not exist over here; I guess that mappers expect people to apply common sense and not make a u-turn as soon as the navigator suggests it, no matter what.
I think that no_u_turn restrictions are somewhat a matter of a local convention. In the US it is common to forbid left turns and U-turns except in a few junctions. In Finland, that practice only exists on some major through-routes in cities.
AFAIK we have no no_u_turn restrictions in Germany, so for me they all look obsolete ;-) Gerd