Yes exactly, its ineffective as an AI. Now lets admit some guys can run 1-2 grams of gear without an AI and they live gyno free. However, if you need an AI and if you select a compound with the intent of running it as an AI proviron is not a good choice. Proviron has its place for some, granted. But as a sole and lone AI, thats sub-optimal at best. Akin to putting out your house fire with your garden hose. A lot depends on ones estrogen sensitivity of course. Myself I can run a gram + without an AI and without gyno. But again if you need an AI get an AI.
RE: mast its better than proviron but again still a second class choice. Now mast as a substitute for test to reduce total aromatization thats a decent plan of attack. That is, instead of running a G of test running 500mg test and 500mg mast. It really all depends on you and what you can and cannot handle. Some guys will get gyno from 400mg of test sans an AI. Know your body and plan accordingly