@w1nt3r
Well by arbitrary I mean more "non-standard". If Ethereum RPC spec had a clause saying you can only query X blocks (where X can be computed from more fundamental parameters like gas limit, block size, etc.), then I'd have no problem with it — at least all RPC providers would behave the same.
This wouldn't stop Alchemy from supporting, say, 100*X block limit. Great for them! But if I wanted to code a universal client, I'd just stick with X.
Today, however, there's no X and I have to empirically figure out X based on whatever popular RPC providers do. This adds complexity