OK, I've done some more reading and I have to admit that I like the setting. Unfortunately the book leaves too many gaping holes. Some of this results from forcing a non-D&D setting to use D&D rules. Here's an example:
The setting has no gods. It has a few very powerful beings (a demon lord, an archangel, some ancient ghosts, the last great dragon, a super-dryad, and more) but none of these (excepting those crazy ancient ghosts) actually grant spells like gods (but some are Warlock patrons). Despite this, there are still Clerics. It very briefly states that they "worship" ideals, but that's about as far as it goes. In a lot of cases, the Clerics are lumped in with Wizards for various Guild roles (like the Lawmages of the Azorius Senate). They made the Cleric option just because D&D has Clerics, but there's no real purpose for them. The only "real" religion--that of the Orzhov Syndicate--it described as a sham (even though it's the one and only one where spells are actually granted to worshipers). So what does the Religion skill cover in Ravnica? I mean, it's there because it's D&D5e, but what does it actually cover in a world with no religions?
My possible fix: I personally see each of the Guilds as being a cult/religion. Azorius worships "the Law," Boros worships angels, Rakdos worships their demon lord, the Gruul worship their totems, Izzet worship their dragon daddy, Simic worship bodily perfection, etc. Sure these religions may not have gods but they each have internal rites & rituals, holidays, and power structures. Knowledge of these needs to be covered somewhere, and while some of them (Azorius, Izzet, and Simic) could be considered magical groups, Arcana already covers a lot.