Reptus is actually a very strong commander. Things about him to note that BRO can't do.
1) He can fling enemies back to his team to be focus fired.
2) He has more innate durability with non stop armor boost from his pike twirl. Blocking 1 attack vs nearly permanent duration armor buff. BRo's active compenent loses its armor bonus VERY quickly, making it a good reactive ability for short trades, but very weak in long fights.
3) Reptus Q resets his Attack animation when activated, allowing reptus to perform 'double attacks' despite a lower attack speed. These when used correctly can make a huge boost in damage. The active compenent also grants a very generous amount of WD, which affects his pike twirl and his ult's damage.
Reptus is more durable than bro. He was meant to be a tankier brawler who had to stick. BRO is more of a short trade brawler. When bro can't kill you before his ult wears off, he becomes far less effective.