I got ~100 northern crayfish from QA last summer for my pond. Primarily to build the food chain for the RES, and the SMB that are going in this fall.

They seem to be doing well. I find dozens of baby crays any time I rake FA. They haven't done any damage to my bumper crop of APW and arrowhead.
It does appear that they are working on the sago pretty hard. I have thousands of small sago bits floating all over the pond this summer.
Last summer the sago stayed intact unless it was disturbed by the rake or daughter's Doberman that thinks it's a Lab.

The northerns aren't bad about drilling holes in the dam, either. I don't see tunnels or chimneys anywhere.

A side benefit of adding the crays has been a notable reduction in water clarity. This time last summer I had ~6' visibility. This year visibility is running ~2'-3',
which is still plenty good. I suppose some of the reduction in visibility can be attributed to the increase in activity in the pond. The laberman dog is in there
several times a day, and we've been swimming a couple or three times a week since the surface temp hit 75°. This time last year the dock wasn't finished,
so we weren't swimming much, and the dog was still a pup and too scared to get in much. So I can't be certain how much of the increase in turbidity is due
to the crays vs. the increase in mechanical agitation, but I like to think that the crays are playing a part in that.