I'm not trying to split hairs on this, but you said error handling is built into WSP and the real issue here is custom error handling. A modern web server is expected to be able to let the sysadmin define a file to use for the 404 response. I realize that 3rd party error handlers exist, but when comparing web servers, WSP does not natively include a custom error handler. So I guess that means IIS also has the feature. However, in the comparison at the above URL, 3rd party add-ons are not part of the comparison.
Native functions are the issue at hand.
The same logic should hold true regarding customizable error handling. I also noticed a few other problems with that comparison. Customizable Error Handing is a big checkbox that ORA overlooked. Several other isues are overlooked or misleading. Try running Perl as an NT user and you will see what I mean.