For readers whose displays are wider than the author anticipated, fixed-width tables cause extremely wide pargins, wasting much of the display area. For readers who need larger fonts, fixed-width tables can cause the content to be displayed in short choppy lines of only a few words each. Many browsers are especially sensitive to invalid syntax when tables are involved. Correct syntax is especially critical. Even with correct syntax, nested tables may not display correctly in Netscape.
Some browsers ignore tables, or can be configured to ignore tables.
These browsers will ignore any layout you've created with tables. Also, search engines ignore tables. Some search engines use the text at the beginning of a document to summarize it when it appears in search results, and some index only the first n bytes of a document. When tables are used for layout, the beginning of a document often contains many navigation links that appear before than actual content. These browsers seem to associate the named anchor with the top of the table, rather than with the content of the anchor.