Well I didn't invent it myself, you can go to youtube and search "fast legendaries" and you'll see the same theories circulating around, that "m0 tour" (all mythic non-keystone dungeons) and lfr count for a lot, than m0 has BLP count per boss while m+ has per chest, it will never be 100% confirmed because Blizzard keeps their mouth zipped about real mechanics of BLP, but I know it worked for me, when I was running more m+ I wasn't getting any legendaries, but when I really wanted to get one and started running stuff like normal EN pugs, normal NH and so on, I got legendaries much faster, my personal theory is that lockout based content counts for more than repeatable content, completely irrelevant what difficulty it is.
Some people claim there is difference and mythic raiding has higher chance than normal etc., but it's super hard to get accurate data especially since Blizzard seems to be tampering with the formula every single patch.
People claim nether disruptor mobs are amazing for BLP purpose, which would debunk my theory about lockout-based content, but I'm yet to hear from a friend that they got legendary from that source or see someone drop one. While in LFR, from world bosses or from mythic0 I've seen legendaries drop so I believe the chance must be decent.
From my 10 legendaries on my main, I got 3 from raid bosses, 1 from m+ chest, 1 from legionfall supply satchel and 5 from emissary caches. And yes, I did get a legendary from m0 on my alt and I saw people getting them dropped in the groups I participated in. I also turned 1,5k supplies on 6 alts last wednesday and 5 out of 6 got a legendary.
M+ is only a last resort after you exhaust all the non-repeatable sources. Of course, if you run m+ for a specific relic or trinket, then you have another reason to do it, however Blizzard wasn't kidding when they said before 7.2 that "they want to get people out of maw", the new round of m+ changes is viewed by many as a "nerf to loot" so definitely as patches progress legendaries drop more, but m+ gets less and less lucrative, first the AP nerf, now this.
Not counting the standard deviation of "luck factor" in obtaining legendaries, so far everything seems to point out people who run more content get more legendaries. Not necessarily "harder content". All these "normal EN 900+ AP zerg" runs? Biggest reason for them isn't even AP, it's the legendary BLP.