A respected youtube channel did a series of the .45-70 inserts on Patreon and tested trapdoor loads in an H&R single shot, a Beretta over/under, and some single shot trap gun. The insert shot well in the HR and Beretta, not so much in the trap gun because it had some issue with the sights and the POI shift. Accuracy depends on what load the insert likes, they seem to prefer lighter 300-400 grain bullets, not the 500 grain behemoths. Groups at 25 yards were about one inch with open sights/bead sights.
All safe to shoot, the recoil is no different than shooting slugs and the pressure was not an issue.
My experiences with adapters in shotguns has been use big bore calibers because if you want the elevation to hit close to the POA on a shotgun using a bead sight or traditional front/rear sights you need a heavy projectile. Windage seems to be on regardless of caliber, but a light bullet like .22 or .32 and probably 9mm is not going to cut it. A 200 grain .38 or .357 may, IDK, what I do know is a 250 grain .45 ACP handload I did hit an inch from the POA at 25 yards and grouped under an inch.
The biggest benefit of the .45-70 in these adapters/inserts is the range of projectile weights; between 250 and 500 grains you are bound to find a bullet that shoots close to a shotgun's POA and in an over/under you might be able to tweak in two different loads that each barrel will prefer and will share the same impact point at a certain distance and make for a very nice, very versatile inexpensive double rifle in the field or you could stick with one insert and have a combination gun.