The trap won't always have water in it. In the winter the A/C condensate drain won't.

But by the time it dries out, the furnace condensate trap will be full.

The A/C trap will always have a positive pressure when the blower motor is running.

Most manufacturers approve that drain set up.

If you plan for the 1 in a million on every install. Then no one will be able to afford to have anything installed.