Just from my experience with playing the Atlantean (from a few years ago!), the advantages its citizen (i.e. villager) has seems to be balanced out by its disadvantages, so overall its effectiveness is about the same compared to, let's say, Greek villagers, but it does have an uphand on certain map types.
- No drop site required (which you already know)
- Faster gathering rate (roughly 2.2 - 2.3x faster than a Greek villager)
- Higher hitpoints (160 vs 65 Greek)
- Higher LOS (16 vs 14 Greek)
- Higher hunting range and attack (14 vs 12 Greek and 12 vs 6 Greek respectively)
- Slightly higher hack armor (30% instead of 25%)
- Faster contruction / repair speed (about 2 times faster than Greek, except town center which is about 1.2 times faster)
- Cost (3x more than Greek)
- Pop (3 pop instead of 1 pop)
- Training limit (allowed 25 instead of 80 like Greek)
- Slightly slower movement speed (3.25 vs 3.8 Greek) This is really rendered moot by the fact that it doesn't need to move much in the first place.
- Takes longer to train.
Stats aside, Atlantean workers have certain strategic advantages over other races as well:
- Decentralized economy - this means raiders would have to go find each individual citizen in order to kill them as they could be spread anywhere around the map.
- More hitpoints mean they have more time to get back to safety when being attacked
- They have a slight advantage over others on maps with sparse resources as they don't need any drop site. On maps with more concentrated resource patches, however, this advantage is vastly diminished.
- Unique to the Atlantean civ, any non-myth unit can be converted into
a hero unit. This includes citizens. This makes them a lot more
valuable and versatile than their counterparts and a viable option
to counter myth units.