I used the Lee trimmers for a long time, but as you get further into the handloading game you may begin to see the restrictions of their system. A problem you may of encountered is that if you lock the stud in a drill to spin, sometimes it's tricky to get the case to spin concentrically on the stud. Anytime you have to mess around getting things right it is costing you time. If you are trimming 20 cases it's not the end of the world if that takes a few minutes per case to get it right, but if you load in batches of 100, it painfully prolongs the process. Cartridge to cartridge consistency can be a problem, but it's more a matter of learning how much pressure to exert on the stem. Concentricity of the case neck can suffer, this is where one side of the neck end up slightly longer or shorter than another. Handgun or hunting rifle loads are quite tolerant to small inconsistencies, but if you are attempting to make match ammo, my rule is that if you can measure it - it matters.
All rifles are individuals, so unless you know the dimensions of your chamber it is hard to say how much you can allow the neck to lengthen. Sooner or later though, it will become long enough to cause an increase in pressure, and accuracy will deteriorate as the neck extends into the throat of the chamber. This condition is potentially dangerous if you load maximum powder charges, but in any case it will destroy your accuracy as a crimping action will cause the neck to bite into the shank and base of the bullet. Damage to the base of a bullet is far more detrimental to accuracy than damage to the nose.
When I used a slow tedious method to trim, after the first firing I would trim well below spec so that the neck would not lengthen enough to be a problem during the life of the case. Now that I have a Giraud, trimming is so fast that it just takes a couple of minutes to run a 100 cases through the machine after resizing, so I trim the minimum each time I reload.
