Generally in this case i would say there will be no significant downside, since the pcb-thickness is only usually around 1-1.6mm. This does not influence the reader.
In most cases there is space of air and an enclosure between the tag and reader, which is a much greater distance than just the pcb
Yes, the tags are equally effective.
Antennas of NFC tags are typically made of coils with one or more windings. When current flows through the coil, it generates a field that goes through the center perpendicular to the coil surface.
If you put anything conductive (like a circuit board) behind or in front of that coil, it will prevent the magnetic field going through the coil. This reduces ...