diabetes simplified
Diabetes is a very wide and complex subject, and needs individual assessment for ongoing aviation fitness in almost every case.
However, for the purpose of explaining to lay people I have found it helpful to divide into three, according to treatment rather than according to diagnostic type.
Insulin treated - no flying medical at all, not even private.
Tablet treated (certain types only) - usually OK for private flying, with little or no restrictions; can be acceptable for professionals, but usually with OML restriction (multi-crew only).
Control by diet alone - if well controlled, can be acceptable for all types of flying, and with little or no restrictions.
Hope that helps !