Tks for the quick answer :-)
I was wondering why sometimes the squad seems to be a bit blind, not finding a target 2 or 3 tiles away from it :-P
But with enough time, another squad will find it and finish clearing everything. I'm now on a bigger map, with 5 assemblers provided by a scheduled train, with radius at 5000, at 99% biter evolution, and they do a great job.
Everything seems to work fine. I love just looking at them clearing a big nest.
A good improvement (I don't know if it would be easy to do) could be to be able on the map view to select a circle (or a square if easier) and tell the squads from an assembler to hunt in this surface, something like "assembler number 3, clear me this area !". The size of the hunt area could increased through a research.