Collectors have the single most powerful unit or character in the game, the ABOMINATION, it is the only unit that can totally wipe out entire teams consistently and it is supposedly a low level unit. No unit inspires as much fear as the abomination. When he blows up you and your entire team die. You can try to kill them with head shots or before they reach you but they often come in such large clusters it is difficult to accomplish.
Preatorian is by far the most powerful boss unit. He is fast. He can kill from range with powerful attacks. He can kill short range with instant attacks. He has high shields and high health. No other boss unit comes close to this combination of range of attack (close and far), total health/armor/shield/barrier combinations, and speed.
Scion is the most dangerous second tier unit that is almost equal to other factions top boss unit. He is the only unit that can kill you with bombard from nearly across entire maps. He has more range and accuracy than even the Ravagers, since he can lob them over cover easier than the Ravager can shoot through cover.
On top of all this, several units at a time can be possessed and thus given even more health, armor, shields, barriers, and offensive power.
So yes, Collectors are pretty op, or cheap, or whatever term you want to use compared to others. Playing them on bronze is equal to Silver of any other faction. On Silver, it is equal to Gold, and on gold it is equal to Platinum, and Platinum is like playing a new even harder level.
If you want a balanced foe or character for that matter, they should not have all the best advantages and few or no disadvantages comparably. For example, they give all the best advantages to the Praetorian, speed, power, total hp/armor, shields, range attack, short range attack. It has no disadvantage. And to hit a Praetorian is a bit harder since he hovers. In the SP, they are pretty powerful, but not as strong as in the MP.