Grinding chains certainly does have a fairly large opportunity to destroy its edge holding capabilities or make it full of hard spots so you cannot file it. That is up to the operator of the grinder, and whether he rushes the job or runs a glazed stone.
Consider the cutter to be a controlled heat treat item. Heated, quenched and annealed to have it in the proper balance of hardness and toughness; my guess somewhat similar in material and treatment of a coil or leaf spring, so a bit difficult to file and probably about 50 points of carbon (medium carbon steel)
The quench temperature would typically be from above 1100 Deg. F giving a Rockwell C of around 60, followed by annealing temperature of around 600 Deg. F. to leave a residual hardness of about 45 Rockwell C. by comparison a bearing would be about 65 and a nail about 20.
Any temperature increase given by grinding that raises the temperature above the 600 degree annealing temperature further anneals to a lower and softer condition. Example an anneal temp of 900 F reduces hardness to 31 RC If that temperature increase by the grinder takes portions of the cutter up much higher than 1000 deg F. (very dull red in daylight) the subsequent air cooling as the stone withdraws, amounts to a rehat and quench and can leave that small area almost file hard at around 58 deg. Rockwell C.
Depending on the volume of metal overheated in the tooth and what maximum temperatures were reached, portions of the tooth could, instead of the design hardness of 45 RC, be at anywhere from a file hard RC 58 to a dead soft RC 22. Typical tensile strength could be then be altered from over 200,000psi. down to near 100,000 psi.
Very thin brittle edges tend to shatter quickly back to a blunter ragged edge if viewed under magnification. Alternately edges that were made soft tend to fold over at the very edge and shed the flap and wear roundish to another inefficient edge. Simply put you can simultaneously soften metal in the cutter and yet random small areas can be left with hard spots, neither one of which is good. I will attach a picture of a cutter that probably failed from being over heated, granted it was ground to quite a thin profile and knots could have been an issue but shows a good reason to be very cautious of grinding on a chain if you want the best performance. That said I still think a keener edge can be had with a grinder than hand filed but you have to be a lot more cautious of heat than most people would ever dream of being an issue.
