Here is my experience, I welcome all to judge as they may. I have flat filed my skis with a file for 20 years, right up until the last 2 years when I found this forum. I have bought several files to do this, including a 16 inch or so file called a cross cut file (I think). It is much more aggressive than a mill bastard and so thick it cannot be bent by hand. For most of that time, I would put the file on the ski, pull it a few inches, it would cut the ptex and clog. I would clean it, and repeat. If doing a small high spot, it would take care of it after many cleanings. So if that is your definition of flat filing, no problem, it works.
When I wanted to do the entire base, I would do about the same thing with longer strokes. In the last 2 years I realized what was happening was the file was clogging with plastic after a couple of inches, the teeth were lifting off the bottom and I was just dragging the plastic along the base. I wasn't cutting much. I would then put the ski up against a wall, which allowed it to bend lengthwise, the file would contact some edge material due to the ski bending, and it sounded like I was cutting so I thought everything was good and I was getting my bases flat. What I was really doing was screwing up my edges and creating a base high ski. I did not own a true bar or know how to use it to check flatness. I have learned a lot in the last couple of years.
Recently, I have tried several files to cut the ptex with the ski properly supported in a ski vice. My opinion is a mill bastard file clogs so quickly that it is not cutting much. Once clogged, the cutting surface is riding off the ski. I did not realize this for a long time. A panzar file has wide, deep gaps between teeth, and they are curved. As the plastic is cut, it has room to extrude out between the teeth and the curved teeth cause it to be ejected from the file. This type of file will actually remove base material with a reasonable amount of effort and not clog as you pull it. It actually extrudes the plastic out the sides of the file. I found it tricky to hold properly so the cuts are thin and uniform. It wanted to dig into the ski and gouge it, especially on the tips and tails. I would say it does work for flat filing a ski, but can over do the cut if not careful.
If we are talking about flat filing the metal edges, any file does a good job and I have found works well. The reason for tuning as I see it is the metal edges wear as you use the ski, (snow is abrassive) and this wear causes the edges to become over beveled and below the level of the ptex. So in relatioin to the edges, the ptex becomes high (it does not wear as much or as fast). You have to lower the ptex first before you can get down to the level of the metal edge to reflatten. If your skis are edge high, then a file works great to remove metal and flatten them. When cutting metal the file doesn't clog like when cutting plastic.
I now have a true bar, a nice LED backlight ($10 at the harware store) and feeler gauges to get a rough measure of base edge angle along the ski. I can check what I am doing to my ski now, and it has "opened my eyes".