I have a Galco shoulder harness for my Sig P239 and am not overly taken by that mode of carry.
I mostly wear a vest and sometimes the straps will be visible through the arm holes, despite the fact they are not that big.
I've also found the weight distribution is just odd enough that my back aches.
If I want deep concealment, I find the UnderCover Comfort tee shirt holster system much more stable and concealed.
So for me, IWB works better for me in almost all situations. When it doesn't work, the UCC shirt seems to do the trick better than other options.
The only case where I still (very occasionally) use the shoulder holster is in certain driving situations considering drawing from IWB while sitting in the car is pretty difficult. But even then, I can't just wear the vest, I need a jacket.