Don't be too hard on the die mfgs. as there's not just one 'correct' answer to your question. To start with, a description of what you're working with, your accuracy goals and how you're testing will be helpful.
-Is adjusting neck tension important for accuracy tuning? Absolutely.
-Is a good full length bushing die a nice way to achieve this? Absolutely.
-Can a full length bushing die
and the bushings themselves give other issues? Absolutely.
In general, my experience has been that with fully turned case necks or necks that have been cleaned up about 75% (no-turn or factory chambers), a
quality full length bushing die does a great job. If the necks aren't perfectly round, a bushing die
may make things worse in terms of run out...the bushing wants to 'self center' (the bushing has radial clearance in the die) on an out of round object (the neck). Does increased run out makes the accuracy worse? Maybe. Maybe not. It depends on the whole package.
I guess that's a long way around saying that the quality of the case necks to be used is what I look at when deciding on a full length die. Even then, you just have to try stuff to see if it helps or hurts.
Even with a dead on chambering job and perfectly turned necks in a real
Benchrest rig, a full length non-bushing die with a fixed diameter neck has some advantages. Think a Redding Body Die with the neck opening honed/reamed to a specific dimension
and a very loose press that lets the whole system 'self center', as one example.
Good basic stuff, well thought out and worked out, is never a bad thing.
Good shootin' -Al