Downunder,
I'm not sure if you mean "difficult to get right" to refer to making or using an arc-style protractor. I doubt you are interested in making one, so I'll assume you are talking about using the protractor.
I don't find an arc-style protractor any more difficult to use than my 2-point style protractor. In fact, I find the arc style easier to use and much easier, for me anyway, to get alignment right.
Using any protractor is an iterative process if done correctly. That is, once you get your second point adjustment you really should go back and double check the alignment on the first point. This is also true of the arc-style, with one big exception. With the arc-style nothing is moved except the arm and cartridge. There is no re-aligning of the protractor for the first point. Unless one can do this re-alignment of the protractor with the exact same precision every time, this change in position of the protractor introduces error.
With the arc-style, you find the correct position for the protractor by locating the position in which the stylus lands exactly on the arc at a point near the spindle and a point near, preferably beyond, the outside edge of the platter. BTW, this can't be done unless the P2S distance is right for the specific arc. Once you have this the protractor is fixed in position. Now when you start the process of aligning the cantilever you are only concerned with that alignment and with keeping the stylus on the line. This alone makes the arc-style much easier for me to use.
Now as for how much time and effort one wants to put into the iterations, that is where the difficulty comes in. But this is true for any method or tool one wishes to use to do alignment. How close is close enough becomes a personal choice. My experiences lately with fine tuning cartridge alignment has shown me there is a huge reward for increasing the accuracy. This is no revelation to LP playback, we all know this. Some of us have found a tool that works even better for us to get cartridge alignment even more precise. Others may have a different tool that does the same for them. As always, YMMV.
I'm not sure if you mean "difficult to get right" to refer to making or using an arc-style protractor. I doubt you are interested in making one, so I'll assume you are talking about using the protractor.
I don't find an arc-style protractor any more difficult to use than my 2-point style protractor. In fact, I find the arc style easier to use and much easier, for me anyway, to get alignment right.
Using any protractor is an iterative process if done correctly. That is, once you get your second point adjustment you really should go back and double check the alignment on the first point. This is also true of the arc-style, with one big exception. With the arc-style nothing is moved except the arm and cartridge. There is no re-aligning of the protractor for the first point. Unless one can do this re-alignment of the protractor with the exact same precision every time, this change in position of the protractor introduces error.
With the arc-style, you find the correct position for the protractor by locating the position in which the stylus lands exactly on the arc at a point near the spindle and a point near, preferably beyond, the outside edge of the platter. BTW, this can't be done unless the P2S distance is right for the specific arc. Once you have this the protractor is fixed in position. Now when you start the process of aligning the cantilever you are only concerned with that alignment and with keeping the stylus on the line. This alone makes the arc-style much easier for me to use.
Now as for how much time and effort one wants to put into the iterations, that is where the difficulty comes in. But this is true for any method or tool one wishes to use to do alignment. How close is close enough becomes a personal choice. My experiences lately with fine tuning cartridge alignment has shown me there is a huge reward for increasing the accuracy. This is no revelation to LP playback, we all know this. Some of us have found a tool that works even better for us to get cartridge alignment even more precise. Others may have a different tool that does the same for them. As always, YMMV.