I view the RA as highlighting unusual risks not normally faced by cyclists. I class cattle grids and level crosses as hazards usually encountered on the road. Likewise livestock, poor road surfaces and steep descents. The one exception is where a busy, main road is used or crossed - I always note these on the RA and explain why I consider it appropriate to use them (e.g. good line of sight, hard shoulder, short stretch etc).
Other than that exception, my RAs are particular localised hazards (either in time or geography). An example would be riders having to ride in areas where there is little if any habitation and possibly mobile phone signals. Or areas where the weather can deteriorate in a short space of time.
That said, my notes to riders (either separately or on the route sheet) will cover these where I anticipate riders unfamiliar with local roads might appreciate some reasonable forewarning. However, I steer clear of words like "Danger" or "Take great care". If you are using words like that I would wonder if one shouldn't reroute to avoid what sound like particular hazards.
Indeed, I completely scrapped my original planned route for my February ride (Redhill Beach Trip) because it relied upon some roads (down by the South Coast west of Brighton) that left me thinking "by golly, that's potentially dangerous". As a result, I've drawn up a completely different route from that I intended (and I suspect will be a much better ride for it, which I will find out for sure this weekend, when I take a jolly little jaunt by bike down to Beachy Head...)
Such actions, however, are less about feeling a duty of care but rather the responsibility for ensuring someone has as nice a day out as I can reasonably put together.