The discussion above is actually a pretty good, nuanced, discussion of the issues.
OSM data is incomplete and sometimes wrong, but so are all the others. OSM’s advantage is that sometimes it is less wrong that the competition.
If you’re a data consumer of any data then you need to be always asking yourself “what was probably meant by …”, and this this doesn’t just apply to OSM. If your bank phones up about two rogue transactions, you need to ask yourself “is this really my bank?”.
If in OSM you see a junction=roundabout without a oneway, or a motorway without bicycle=no, you can as a data consumer say “I will assume that this is oneway” or “I assume bicycles are not allowed” if you know the jurisdiction for which you are making a map, you can guess the “missing” tag (and with motorways at least, that might depend on where in the world you are).