What are the main differences between rabbanut mehadrin, and, let’s say, Rav Rubin or Bedatz?
The main differences between the different kashrus boards are in the level of supervision (concerning ingredients, concerning the separation of terumah and maaser, and so on), in chumros applied to the shechitah process, and in production of foods in general. Bedatz (Eidah Chareidis) is perhaps the most trusted hechsher — they’ve been around for a long time (80 years) and have made a good name for themselves — but there are certainly other good boards that can be relied on. The level of the Rabbanut Mehadrin boards depends on the individual Rabbanut — some are well reputed and widely relied on, whereas others are less well reputed. Rabbanut has an advantage in being a public service with set standards that won’t change because of financial considerations, but a disadvantage in that they are catering for the entire public, and cannot demand the highest standards. Rabbanut Mehadrin is of course by far superior to regular Rabbanut, but in general still doesn’t reach the standards of Eidah Chareidis (for example).