A local plumbing company (big name) said that anything draining from upstairs would suck my p-trap dry. I asked about the mechanical vents, and he said it would not work. He went on to tell me that connecting was not advised and he would not accept the work.
Basically, I just want to tie into the 3" drain from upstairs with a sanitary Tee for a small sink. Adding the mechanical air vent before the P-Trap should eliminate any concerns around draining it, no? He also said if there was a backup, I'd have waste water issues, but isn't that the same for any other drain further down the line (like my master bath)?