Through some quirk of internetting happenstance, I stumbled onto Michael Mah's video on how he made his own concealed carry holster. So, while I was waiting around the rehab/hospital system for my paraplegic Son's surgery and recovery, I used the time to make a holster of my own... sort of like a type of therapy for my mind.
The first one I made was a holster for my Judge, patterned after the old west style complete with cartridge belt. I used merely a leather thong for retention in it. I don't know what it would be classified as... probably a 1.
Then I made three more.
The second holster was for carrying my Bersa .380 in the middle of the back, OWB. It didn't work well enough, as the leather I used was too thin, and I'm too old to twist myself around for two-handed reholstering.
I have since made two more holsters for my Bersa .380 to be carried at 3:00, OWB. Each one has gotten a little better. They are hand stitched using woven 30# test braided fishing line. I used a deer antler to bone the last ones. I don't think I boned them enough, after seeing some professionally made boned holsters; but they're good enough for me. One of them is still almost too tight, but it's getting better.
All that said, I believe that, as SGB wrote, if you have a good boned holster for concealed carry it should work very well, retention-wise. If you're going to open carry, I think you'll need more retention... depending on the location you'll most likely be in.