Matchups like Lendl-Wilander imo show, how difficult it is, to determine level of play and to define unforced or forced errors. Both played more on long term strategy, a sort of position war, Lendl a bit too defensive for my gusto, Wilander 1988 more willing to attack on certain points. A match is not decided by the sum of games or points, but by winning the decisive quality points, there are big points and minor significant points. And still the point on the same scoring situation could be more important in different situations. For example: A 40-15 point for a server has different worth than in a 15-40 situation, if the returner is leading.
To the unforced error question. In such a match Lendl- Wilander you could bring up the hyothesis - i now make a too radical argument to get the point over - that they played no unforced errors at all. The same shank is a different error, when hit after one excange or hit after an excange of 30 strokes and more, when the player is near to collapse. So in a way, in such a long match with many extremely long exhausting rallies, many more " easy" appearing errors are "forced" (by nature, exhaustion, mental fatigue and other factors) than in other matches, where a shank is a shank, by lack of concentration, bad positioning, bad stroke production or other pure technical or form factors.