I can clarify the problems with Fulltext MySQL on large boards. MySQL only supports fulltext indexing with MyISAM tables. MyISAM uses table-level locking (so only one operation can happen on a table at any given moment). This can bog down significantly on posts with dozens of simultaneous users.
Large boards like phpBB.com use InnoDB tables, which do not support Fulltext indexes. But they does offer the performance boost of row-level locking.
For boards of a million posts or so, the best search mechanism is probably Sphinx, available at http://code.phpbb.com
. It is not supported to the same level as the rest of phpBB, though.
The next-best option is Fulltext Native because it does not require MyISAM tables.
MySQL 5 is more efficient at fulltext searching than phpBB is, so for moderately sized boards running on MySQL 5, MySQL fulltext is probably the best method.