Statistics are hard to come by, but...
It is estimated that there are more than 40 million people in the world who are blind or visually impaired.
In addition to people who cannot see the screen, consider those people who struggle with learning disabilities. They may have difficulty associating the data in complex tables with meaningful headers, especially if the table is large and complicated.
It is estimated that more than 15 million people in the United States have some form of learning disability.
What types of tables are there?
NOTE: Regular tables should be used whenever possible. Avoid the use of non-standard or irregular tables. Simple tables should be used whenever possible. Avoid the use of complex tables, even if they are still considered "regular" by definition.
Therefore, the best possible practice is to use regular, simple tables.
For example, irregular tables include those that do not have header information in standard positions. Standards show that column headers should be directly above the data they represent and row headers should be on the far left of the data they represent.
Complex tables, while they can still be classified as regular, still increase the chance that people will not have an easy time comprehending them. People may have still difficulty accessing the information. The overuse of rowspan and colspan with nested headers can cause this. The author's recommendation is to keep rowspan or colspan to a minimum.