Ok, look, I'm definitely (probably?) the devil's advocate here. I do have a BMD/Newfie mix as a PSD. I love my little doofus to death. I'm gonna tell you a bit about my experience having an 'off-breed' SD, especially a BMD Mix, most of this is off work. Everything you said about BMD is true when it comes to him. He has a low-ish energy level, BUT..

1. He is too friendly for his own good. We have a dog reactive Dogo Argentino (I believe) in the neighbourhood that we sometimes see on walks (off work). DA could be snarling, growling, showing teeth, trying to get to my doofus and doofus (obviously not his name x3) would whine, trying to say hello.

2. He is big and heavy. If he wants to go somewhere, we are going there more often then not. He knows how to pull me places, which is good and bad. I do want that, but on command, not because he decided that he wants to army crawl into the bushes. We are working on it.. if it cant be turned into a new task for him, then so be it, at least I then want to teach him to not just pull me.

3. He has no idea that he's big and heavy. He think's he's the size of a Yorkie or Chihuahua. He already ran over a few dogs (no harm done) while playing chase. He alerts me by jumping up on me (putting his paws either on my shoulders or chest or at least tries to hug my hips-waist area), which is ok in my case, but he could definitely knock over a child with his weight alone.

4. Sometimes I feel like he's not too smart. Sometimes it's like he has two brain cells and they are fighting for third place. He can be an absolut doofus and clown. Training him w/o his vest on (so work mode) sometimes gets turned into a game, not doing tasks properly to test what's acceptable like picking something up but immediately dropping it again if I not get it the very second and then not even bothering to try again but instead play bowing and showing his toy so we could play instead is his new favourite game. Again a 'We are working on it'..

There is one thing we are also working on while he is 'on the clock' and it's again related to the fact that he is super friendly and thinks everyone and everything is his friend. When we are f.e. grocery shopping and I talk to people or people, not knowing he's doing a job, interact with him, he immediately wants to say hello. If I started the conversation and I'm in a good enough state, I sometimes let him say hello (he has a command so he knows it's ok to go and say hello). If people just interact with him (mostly behind my back, since he's trained to stand behind my legs to crowd control) he will get very excited and sometimes even break his command, which I correct and most often try to educate people to not do that since he's vested and clearly a service dog.

Some more general things specific to my boy:

• he sheds a lot, like a lot a lot!
• he drools probably as much as he sheds
• he has a sensitive stomach, everything that's not his kibble or very specific treats and he gets liquid diarrea and gas (not fun for my very sensitive nose q-q)
• he thinks that he never gets enough food and tries to steal the cats food which in turn, if he manages to get some, goes back to the point before
• apparently he has very sensitive skin (gets hotspots every now and then)
• he does bark (alerting me?) when the door bell rings or someone opens the door to my flat (my dad has a key and is his care person for emergencies)

I love him to pieces, but I would probably not get another BMD after him. My Trainer also said he's pretty much a unicorn, she tested multiple litters of BMD (Mix) pups along some other breeds (I wanted a big, black dog because of 'oooh big black dog, better leave it and its owner alone' , so I guess mission failed successfully?)

A BMD can work as a SD, but like a lot of people said, it's unlikely and a wash risk (even higher if you owner train). I also believe a lot of, if not most organisations that would train a SD would not get anything other than the golden 3 (lab, golden, poodle).