The integer quantum Hall effect (IQH) demonstrates the robust quantization of longitudinal conductance of a 2D electron gas under a strong magnetic field. The theoretically predicted quantum anomalous Hall effect (QAH), which is the IQH counterpart with ferromagnetism taking the role of an external magnetic field, was experimentally demonstrated in 2013 in Cr doped (Bi_xSb_x)_2Te_3 (BST). However, the onset temperature (30 mK) is 2 orders of magnitude lower than the characteristic ferromagnetic critical temperature. The picture often used to describe QAH is this: a 2D topological surface state (SS) exists between the bulk conduction and valence bands. A Dirac mass gap opens when magnetic dopants e.g. Cr are added to create ferromagnetism. Due to its topological nature, a chiral edge state appears when the Fermi level is tuned into the mass gap. Using millikelvin visualization of SS Landau levels, we probe the spatial heterogeneity of the electronic structure due to Sb and Bi intercalation in both BST and CBST, hence discovering that Bi atoms cause a strong shift in the SS Dirac point which is independent of the disordered mass gap caused by the Cr atoms. In consequence, the electrostatic disorder randomizing the Dirac energy and magnetic disorder independently randomizing the Dirac mass gap conspire to drastically suppress the minimum energy gap into the range below 100 micro-eV for nanoscale regions separated by less than a micron. Thus our study reveals what fundamentally limits the fully quantized anomalous Hall effect in Sb2Te3-based ferromagnetic topological insulator (FMTI) materials to very low temperatures.