In the past I've also looked at that area but mostly further east on the Trans Taiga. No special reasons for not paddling there, just the usual issues which include crazy bugs, not particularly exciting scenery and probably areas where the shoreline isn't great for camping given that much of the natural shoreline of Lac Sakami is underwater since being incorporated into the James Bay Project.
If you are looking for a large area of flat water paddling "in, around and out" style it would probably be a good experience. I doubt you will see many(if any) people which would be the whole idea of going that far north.
I would recommend mid-August as the best time, fewer bugs and lower water levels would likely open up many beach camping options.
If you check this Surficial Geology map:
https://geoscan.nrcan.gc.ca/starweb/geo ... 1=R=205040You can see that most of Sakami is surrounded by peat bog with some sand/gravel areas. if you travel to the southeast section you may find some exposed rock.You will also notice it is in a small area that is devoid of eskers which leads me to think that off the water exploring opportunities are very limited. I've used this map for several trips and found it useful and reasonably accurate for determining the best potential locations for camping and/or hiking.
My guess is that the local Cree mostly travel the area in winter, I've paddled similar areas in Labrador, fly-in fishers are about the only people there in Summer, the locals only access the area by snowmobile and question the judgment of "foreigners" who want to go there during bug season.