I’ve seen a lot of talk on forums over the years about “Balance” with regards to Specialty Priests. In general, most of the arguments, in my opinion, are ridiculous or (at best) looking at the issue in the wrong way.
The arguments tend to frame it as if all priests should be equally appealing to players, with equivalently powerful abilities at specific levels, and similar sphere distributions. But why should that be the case? Gods can represent a wide variety of portfolios, some of which work well for adventurers, some of which do not, but are by no means less crucial to the fantasy society they are designed for. Gods of fire, magic, luck, and healing will probably be more appealing to players and more useful on adventures than gods of peace, architecture, or history. For that reason, a DM creating specialty priests for the latter powers should not feel they have to choose specific powers that will draw players to want to play them. As adventurers are only a small subset of the population, those other priests would be quite useful to society as a whole, in addition to being useful to adventurers as NPCs between adventures, and their powers should reflect what the populace would look to them to do regularly. And, of course, some players may want to play them, just for the added challenge; there’s nothing wrong with that.
Besides some portfolios being better aligned to adventuring careers, certain portfolios align better with certain spells or abilities. For example, deities of music or love may get access to the power to charm person, but a deity specifically of enchantment/charm magic will probably get charm person earlier or get more daily uses of it, because the spell more closely aligns with the deity’s portfolio. Is this unbalanced? Not at all. It takes into consideration what the deities represent. One should expect that a deity of a very specific subject matter would grant priests greater powers over that subject matter than a deity that is much more generalized, or a deity whose portfolio only touches upon the matter. Similarly, the former two gods may get some other enchantment/charm spells from the wizard spell lists, but the latter deity, as a power specifically of that type of magic, may open that whole school of magic to his priests. Meanwhile, the former two deities would grant abilities that the third would have no access to at all. In fact, having more spells available to a priest can be a curse in disguise, as they have harder choices to make when praying for spells, and may be more likely to take specialized spells that turn out to not be very useful in upcoming situations. Usually only ample time to prepare and knowledge of what is to come truly turns a large spell selection into a significant advantage.
Of course, that’s not to say that a specialty priest of a god of meteors should be allowed at-will use of meteor swarm at first level. There are, after all, multiple types of game balance, and that would clearly violate one of them. But there’s also no rule that says certain powers, such as meteor swarm, can only be granted at Nth level; with suitable restrictions, or with greater usage or flexibility, powers can be granted at a variety of levels without causing problems with gameplay.