Maverick subclass kinda 'meh'?

Okay, so at level three the Maverick can make one "prototype" which is... a device that stores a single spell? And just one per day?

Maybe it's the extreme open-endedness of this prototype-making power but I'm not really sure what you're generally allowed to make. 

A clockwork rabbit that hops onto the battlefield before casting Faerie Fire? An umbrella that contains one cast of Thunderwave before blowing apart? An ice cream maker? A ballistic missile? It all just seems like a really complicated way to gain a spell slot.

Or is the bigger benefit having (limited) access to the full wizard and cleric spell lists?

The fact that the spell doesn't have to be one you have prepared, and can be anything from the Artificer, Cleric, or Wizard lists (encompassing the vast majority of spells that exist) is a pretty big deal.

Also note that you don't need the material components for the spell even if they're consumed or have a cost. 

At very high levels this feature also allows you to cast 6th-level spells, which an Artificer normally will not be able to do.

It's also worth looking at the variant capstone feature for the Maverick, which is basically a narrower version of Wish, something that's also normally outside the reach of half-casters.

Good point! Ignoring a consumed material is pretty huge, as is the ability to cast level 6 spells later on!