I am very much a fan of barrel tuners and Ive felt this way since 2017.
I have done a lot of testing with tuners of the 'moveable weight via a thread' design and there is no doubt in my mind that they can help a shooter in reducing the average size of the their groups in many different scenarios. The tuner could be connected directly to the barrel or as part of a moderator/tuner set up or a brake/tuner set up, they all work the same and deliver the same results.
The problem regarding their effectiveness lies very much with how much effort and understanding shooters put into them, the more you understand the more they reveal the benefits. The benefits could be different things to different people though. One very obvious benefit is being able to tune ready made ammunition such as factory ammo, they do this very well and thats why very few self respecting top level benchrest rimfire shooters would be seen without one. Apply this to factory CF ammo and you can see some real improvements within the same batch. Time when the bullet you want to use isnt accurate enough can be minimised within reason.
I have loaded rounds for just velocity with no regard for optimum powder charge then tuned the load to shoot as well as a load developed using close observations in powder charge weight. The benefit of this is Ive found accuracy at the velocity I wanted and not where the node was which sometimes can be well below the speed you wanted with the next node being too high causing pressure issues.
Something Ive never been able to do is tune a barrel with a barrel tuner to be any more accurate than a well tuned barrel using any of the more recognised methods.
Once a barrel is in a high state of tune with any tuning method I think thats it, there is little if anything to then be improved upon, highly tuned is highly tuned regardless of how you got there. You also reach a stage where other factors like how well you can shoot comes into play for many people or how meticulous in your testing, record keeping and data analysis you are, being able to discern very small differences becomes difficult. I used to have a very stable shooting platform in the form of a 46lb bench rest heavy gun, that allowed me to see things I had never been able to see shooting my regular rifles but it cost a lot of money in time and testing to find.
I also think some tuners are better than others, the Bramley one is very good. Some are just too big and too heavy where in combination with the thread pitch and increment sizes they make for too coarse an adjustment and you miss lots of stuff in between.