This is my take on in line seater dies.
Seater dies are a must for accuracy especially for high power jacketed rounds or GG lead bullets. The problem with a lot of in line dies is the inaccuracy of the threads cut in the dies that hold the seating stem. If that seating stem is out of line when it is locked in place because of loose threads or just a plain miss bored and threaded die the stem will not seat that bullet straight.
I have use long stem ball dial indicators to check a dies stem to see if it had run out and it was off .0038". I sent it back for a replacement and the new one was off almost .002. I sent that one back for a refund and got a Wilson.
A in line seating die that is not made for a specific case /caliber that does not support the case like a chamber has to much chance for error with just using a faulty case holder or a dirty one or ram misalignment with the die taking loose or faulty threads holding the stem.
A seating die like the one Shiloh makes for the BPCR that is cut using a chamber reamer is the best way to go in my opinion.
How straight that seating die is mounted in a loading press with the loose threads those presses have can contribute to a misaligned die that does not support the case wall.
For a PP bullet if they are seated way out the problem is solved with out a seating die if the bullet has a proper bore fit.
Kurt