Give or take, due to different lenses and technologies, it has been proven that equivalence works. It is just that it doesn´t because a) the lenses, b)you don´t have equivalent setup for the smaller system. 35mm f/1.4 at ISO100 on FF means 22mm f/0,88 ISO62 on Canon APS-C, or 17,5mm f/0,7, ISO50 on MFT. Many MFT cameras start at ISO200. So you are already two stops into amplification to get the same image brightness as from FF, and there is nothing you can do about it. Then obviously, small lenses do not scale with resolution. The smaller these are, the less overall resolution they can transfer (again, all else being equal). There is no consumer grade small lens, that can resolve what FF or MF lenses can.