There are two problems, one of which you covered, and the other you didn't.
If you want to find out the Return on Capital Employed, you need two things, you need to know the "return" ie profit, which can be subjective, and you also need to know the "capital employed".
You dealt with the problems in calculating profit, so I won't go into any more detail on that. But what about the problems in calculating the capital employed? If you travel on an airline for example, you might think that the huge plane that you are sitting in is a large part of the capital employed. After all, you can't run an airline without planes? The problem is, these planes are not on the airline's balance sheet, or very often, on anyone's balance sheet. David Tweedie, chairman of the International Accounting Standards Board, has declared that one item on his bucket list is to fly on a plane that is on someone's balance sheet. Companies do not like putting assets on their balance sheet, because that increases the capital employed, and therefore reduces the return on capital employed.