Since this is in the hydraulic forum I presume you mean hydraulic fittings. I use tape all the time on water/air pipe threads.
Many folks believe, quite correctly, that tape can find its way into the pump and filter and really gum up the works. Paste does not do that as it does not have what amounts to the filament threads. I believe that an argument can be made that tape is just fine but if not installed very carefully and more importantly IMO, when the fitting is removed you have to be very careful to not leave any of the remnants so that they can get down into the fluid and find their way to the pump/filter. Not always possible or easy.
The only time I have had paste leak was when the thread had minor damage to it and in a 2800-3000 psi system that needed to be replaced anyway.
Best idea is to not use NPT fittings and the problem goes away. JIC, ORB, ORFS are far superior in just about every way.