Install Joomla 3.x in a sub-directory - yes or no?

Recently, a 'web expert' was asked to review one of my sites and among the comments they made were:

"Hopefully this site is placed in the sub-directory and not on the root of the domain, as Joomla is hackable. If this is not done it can be done by htaccess files in root and the sub-directory, lastly modify the config file, if need be for the base url."

Is there an advantage or benefit to installing Joomla 3.x in a sub-directory vs. the root directory like the instructions say? Does it add security? Help please.

It is certainly not common practice nor adopted security pattern , but on little tough it could have security benefits because you are moving items of attack from place shorter targets so in some cases it could null effect of attack or in some cases confuse malicious code while executing. But also I think not much so I would not go for it because I think it is not worth of trouble and other problems it may produce. It could seriously decrease your site SEO.