Druids can’t really use weapon familiarity, while they can use weapon proficiency, so on some level you’re giving the character “something for nothing.”
On the other hand, a dwarven fighter would have exactly the opposite position, having no use for weapon proficiency she already has from her class, while the weapon familiarity could potentially allow her to use weapons she otherwise couldn’t.
Ultimately, classes that lack martial weapon proficiency usually aren’t expected to make a lot of weapon attacks. The only real exception I can think of is a cleric. Druids often make a lot of attacks—but with tooth and claw, not weapons. Since wild shape replaces a druid’s physical ability scores, they often have low Strength and/or Dexterity in their regular form. If they do, then they won’t be very good at weapon attacks. If they choose to get more Strength, then they are sacrificing a major benefit of the druid class.
Similar arguments can be made for most of the classes without martial weapon proficiency.
So I tend to think that weapon familiarity is the more powerful feature, at least in theory. Dwarven weapons available that way are pretty poor—as, indeed, exotic weapons often are—but at least those who benefit (those who have martial weapon proficiency) are those who are likely to use it (likely to make weapon attacks as a major focus).
In short, yeah, trading weapon familiarity in some exotic weapons for weapon proficiency in some martial weapons is probably fine in general, and certainly seems like no problem for a druid. A dwarven cleric might be more of a concern, but ultimately both weapon familiarity and weapon proficiency are both really low-value features, so I wouldn’t be overly concerned about it even then. Ultimately, either of these amounts to an average damage roll just +1 greater than without it, for the appropriate classes. Even at 1st level, 1 damage is a paltry amount. It swiftly degrades in value as levels are accrued. Really, the only martial weapons that offer a significant improvement over simple weapons are the bows—you know, the ones that elves are already offering racial proficiency in.