Meh, Praetorians are pointless. On Horizon its essentially you and about a bazillion stacked boxes and other items of cover so you get to run around in circles for ages slowing killing it. On the Collector Ship its basically the same thing except you need to kill the few Husks/Collectors that attack you first and theres less cover but a nice big square path for it instead.
The whole "slam on the ground and regen barrier" is a pain though.
Harbinger not much of an issue though unless you let him be 1. Except the ambush on the Collector Ship there really isn't much of a threat from him. He's mostly just annoying because he ALWAYS possesses the Collector thats about 1 bullet from death.
The other thing is his biotic attacks sometimes ignore cover.