Like many late game "buildings", Sewers are underpowered for what they cost.
Alternatively, Sewers (and Neighbourhoods) could both add Housing and Amenities. Maybe Sewers give +2 Housing, +4 Amenities, while Neighbourhoods give +2/+6 Housing, +2 Amenities. Getting both Housing and Amenities at the same time would better reflect the overall impact of these additions to city life, result in a bigger immediate pop boom, make the extra population more supportable (and therefore more productive), and provide a better return on investment for their cost.
Amenities for Sewers and sewer systems makes perfect sense: quite simply, you cannot live - and stay alive - in a city without a sewer system of some kind, as the modern 'mega-cities' and their slums are proving and Rome could have told them in 600 BCE!
On the other hand, Neighborhoods do not necessarily give Amenities: throwing up tenements (or Roman Insulae) gives you Housing, but it doesn't intrinsically do anything to make that housing attractive.
Suggestion: Neighborhoods in attractive enough locations also give Amenities.
So, a Neighborhood in Charming or Breathtaking tiles gives, respectively, +1 or +2 Amenities in addition to the extra Housing: people will pay big bucks for the view, or Ocean Front, or clifftop high above the clamor of the city...
Neighborhood might also need a rework of its Buildings, because right now, 1 each Food Market and Shopping Mall in a city no matter how big giving a whopping +3 Food or +1 Amenity, respectively, are a total waste of the time and effort it takes to build them. To take a leaf out of European and Asiatic cities as well as just American, I suggest Neighborhoods could have for:
Additional Food:
Street Market (Industrial Era)
Super Market (Atomic Era)
Additional Amenities:
Neighborhood Park/Biergarten (Industrial Era)
Shopping Mall (Atomic Era)
Another possibility, if there is only going to be one 'Neighborhood' building allowed per city, have the amount of Food or Amenities produced by, say, the current Food Market or Shopping Mall not be fixed, but based on the number of Neighborhoods in the city - say, +1 Amenity per Neighborhood from the Mall and +2 Food per Food Market, so it scales at least a bit with city size.