Okay, they aren't much different from other contemporary versions of the druid in terms of how they play, but the process of getting there is freaking terrible. On the surface, the idea of having a sub-class of the cleric as a kind of "prestige class" would seem to work. After all, just as a magic-user might conceivably go from studying all of magic broadly to studying a single "school" in great detail (although I haven't seen a version of D&D that allows this, except for 3.5 and 5th edition where all wizards choose a specialization), the cosmology of Mystara might allow a cleric to switch from worshiping a larger pantheon to honoring a single Immortal from that pantheon.
Unfortunately, this logic doesn't hold up when the new focus of worship has little or nothing to do with the old one. Druids' entire nature - the thing that makes them more than just (in AD&D 2 terms) another mythos priest - is that they worship the unified force of the natural world, instead of personifying it the way that the standard cleric worships a God or Goddess of Time, of Magic, of Death, et cetera. In real-world terms, this shift would be like a Roman Catholic priest who is credited with reviving interest in the Church (thanks to his* reputation as a miracle worker) deciding, after twenty years, to convert to a pantheistic Neopagan religion.
Even worse than the complete hash this makes of Mystara's already unusual cosmology is the fact that the BECMI/Rules Cyclopedia druid is locked to clerics who have already attained name level (9th or higher, in this case). I have yet to have a party in any RPG I've run reach 3rd level, let alone 9th. Hell, I saw getting one of my AD&D players' character to 2nd level as cause for celebration. It's the same problem I have with the bard class as presented in AD&D's first edition; in a day and age where a twelve-year-old video game still has at least twice the number of players as even the newest (5e) or most popular (probably 3.5e) edition of D&D, very few people will get to a high enough level that they'll be able to play this type of character. It's one thing to make a class exceptionally difficult to qualify for due to ability score requirements - it's quite another to make the player's character jump through a number of almost-literal hoops over who knows how many months or years to play their chosen class... assuming the character doesn't get killed in the process.
I would normally just shut up and use the druid as presented for OD&D in Eldritch Wizardry, but this is the very first presentation of the class - and it really, really shows. The distribution of powers is far from equally spaced out.
As for why I've decided to write about this? I'm thinking about adopting the Rules Cyclopedia as my preferred rules set for non-Advanced D&D (for a game I'm hoping to run over the summer), but this is one of the things about it that really annoys me. My ideal goal is to find a game that I can run completely without house rules, and so far B/X is the closest to that goal.
*Yes, "his". I'm not using it as a lazy, supposedly gender-neutral pronoun like the D&D books in the 90s; I'm simply following the real world in this example, as the Roman Catholic Church still refuses to allow women to become members of the clergy. Sad, but true.