Bottom line is, as a competent driver, you should be able to drive to a speed appropriate for the road and conditions without unduly hindering or obstructing other road users. If the road was clear, there was good visibility and the weather wasn't anything unusual, then a driver should be able to drive to the national speed limit for that road; even if they are slightly cautious for whatever reason (not familiar with the road for example) - driving at 50 would be reasonable - but 30 would, in my eyes, be considered far too hesitant and therefore no safer on the road than someone driving too fast for the conditions.
It is possible there was a problem with the car - I have had to limp my car home at a slow speed - however I kept pulling over and displaying my hazards whenever I did so, so that a) people could see there was a problem and b) I didn't hold people up or cause an obstruction. I remained aware and conscious of other drivers around me.
Drivers that pootle along at a speed FAR too slow for the road/conditions, completely oblivious to other drivers around them, ARE dangerous because they are not driving with due care and attention and are unlikely to be able to react properly to anything unexpected.
When I took my test, I was told that you were expected to drive at an appropriate speed taking into account the national speed limit for that road, the road conditions, weather, other traffic, visibility etc, and that you should not be holding up other drivers. If someone was simply driving at 30 in a 60 because they can't handle the road, then yes - that IS dangerous.