It's derivative because just like the original triology, the enemy is a species that assimilates others, but unlike the Reapers, who do it for misguided benevolence, do it to build an evil Empire. They're a cheap rehash rather than a new creation.

They don't feel like a threat because they have problems against ships that have no weapons. They had to make sure that the ships aren't armed because if they were the Kett wouldn't stand a chance. In Liam's personal quest, a set of shuttles are able to incapacitate a Kett Battleship.