How well did the League of Nations preformed in Aaland Islands and Bulgaria cases?
While people can, of course, have different opinions on how well the League of Nations did in these two cases, the two cases are generally seen by historians as examples of what the League of Nations did well. Historians, then, would typically say that the League did well in these two cases.
The League of Nations was set up in large part to allow nations to settle disputes among themselves without resorting to force. In both of these cases, the League did just that. In the case of the Aaland Islands, there was a disagreement between Sweden and Finland as to who should have control of the islands. The League disappointed the islanders in that it did not let them have self-determination. However, it did strike a deal that allowed everyone to be relatively satisfied. It allowed Finland to keep control of the islands, but protected the cultural rights of the Swedish inhabitants. By preventing a conflict, the League lived up to its mission.
The League did the same in the 1925 conflict between Bulgaria and Greece. In this case, Greece actually invaded Bulgaria in reprisal for the shooting of a Greek soldier. The League was called in to arbitrate and ruled in favor of Bulgaria. Greece was not happy with this, but it did eventually comply with the decision. It pulled out of Bulgaria and paid the compensation.
In both cases, then, the League was not completely successful in that it did not make everyone happy. However, this is not really possible and so historians say the League did well in both cases because it prevented armed conflict.