When I am carrying luggage I prefer to use the Roissybus, and it is a wild ride with those reticulated buses through the narrow streets. I am amazed at the skill and judgement of the drivers. For a short sightseeing trip without luggage I would definitely use the RERB. It’s quicker, more reliable, drops off in a better sightseeing area. There are pickpockets everywhere and you have to be prepared and alert. I wouldn’t see that as a strong reason to avoid the RER.