Trajan was a good military general, and IIRC a reliable historian (a good source of information of the Julio-Claudians and so forth Romans before him). But he did over-extend the empire, which is why the next emperor Hadrian had to give up some provinces, and greatly fortify others. He was also incredibly lucky to rule at a time when there were no great threats on the borders - the Germans were relatively settled, the Scythians / Sarmatians were friendly, Parthia was occupied on its other borders, and the Bedouins and Berbers were not highly interested in raiding the southern provinces. So in short, he did do a couple of things good, but a lot of that was due to being in the right place at the right time.
