An assumption, but probably an accurate one. These guys do want to stay as far as they can from any liability (and frankly, I can't blame them any in today's business/litigation environment). Have you SBN'd a Defiance Turbo?
I agree, you can't blame the action manufacturer at all. The SBN process is out of their control, but yet they'd be the first entity sued if something bad happened.
No, I haven't done a Defiance. I've done an FN SPR and a Savage Model 10.
I was able to directly confirm the Savage was 4140 steel, but I had to "read between the lines" to determine the SPR was the same (4140). I have a good understanding of metallurgy and heat treatment, so after measuring the hardness of my actions and looking over the phase diagrams of 4140, I decided to give it a go. So far so good. One thing I was worried about was the concept that bolt lugs and receiver lugs should be of a different hardness spec, or they are prone to galling. I am not seeing this at all, and I have ~1500 rounds through the SPR, and ~500 through the Savage. Everything is good to go.
I didn't measure/indicate my Savage pre/post-SBN, but I did for the SPR, both before and after, and didn't have any (detectable) dimensional change.
The SPR action cycled smooth and nice before SBN, but the Savage was - well, a Savage. Not "bad", but not sweet either. The SBN made the FN a little nice, but really improved the Savage a lot. The finish is extremely scratch resistant and very difficult to get it to rust. The barrel on the FN is stainless, and it has a few freckles of oxidation, meanwhile the action is totally good.
As long as the metallurgy is agreeable to SBN, I'm 100% for doing actions.