The main factors which resulted in the success of the roman army:

The roman soldiers were treated harshly and mistakes were not accepted. The roman soldiers were very dedicated to their job as they feared the consequences of their mistakes.
For example:

-If a legion didn't perform to the correct standard their food rations were reduced.

-If a soldier broke any of the rules they were flogged infront of their fellow soldiers

- A soldier did not wear any back armour as they weren't expected to turn their backs to the enimy.

- They lived in barracks under strict discipline