It's hard to tell how ubiquitous using a sewer for waste disposal rather than simply throwing it outside was, since a lot of accounts come from those with a vested interested in making Rome look good/bad. However, there were laws ordering compensation of those who the throwing of waste injured, so it was unlikely to be as sanitary as a modern city.
I an supprised that there could be much controversy over Roman sewers. There are preserved Roman streets across Europe and the sewers still exist today for us to inspect. The physical evidence is there in hundreds of sites. Of course their standards of cleanliness were different than ours. They had a system of collecting urine from each home so laundry services could wash clothes in pee! (It was a good source of ammonia.)
