Another common problem is the complete lack of off-page SEO, like no backlinks no substitution, no nothing. This will get you nowhere as you website will be almost invisible in the search engines without some proper off-page SEO.
Of course all these services can be learned or bought directly from freelancer that offer such services.
I would say the most common problem websites have with SEO is over optimizing.
Maybe it is just the websites I deal with but it seems like people either completely over optimize their website which throws a huge red flag in the eyes of Google, or they don't do any SEO at all. Very few websites have a happy medium.
Most of the problems to the website is keeping it up to date. In, seo a website needs to be updated by content and navigation point of view so that user may get some valued information.
That is a unique question, I think there are hardly any disadvantages associate with doing SEO for your website except a small thing that if you have a blog of your own and any precautions to block spammers is not taken then spamming activities can harm your website.
Multiple homepage version, Query parameters on url's, Broken site maps.
