On average, I would say many players don’t get legendary drops or finds during the playthrough (i.e completing the story line mission).
I took forever in leveling my main characters mostly to stop and farm for tons of cash, and to farm the vendors and chests. I always wanted to have lots cash available in case I saw a legendary for sale.
So that’s how I got most of my legendaries as I ran the playthrus, thru the vendors. You can also collect purples and grind them for a chance at a legendary.
IMHO, I think there only a few bosses who are reliable in dropping their dedicated legendary when farming them during the playthourgh. But one or two you have to dashboard to farm and you may also wanna save their missions till LV50.
And you can’t count on the chests for legendaries, but they’re good for selling the stuff for money and collecting blues and purples for the grinder.
Well, Good Luck.