Many CMS developers are using this as a selling technique to promote more sales. There is never a CMS that is totally SEO, there is always some work that needs to be manually done in order to make it SEO. That means if you choose WordPress and expect the site to rank on the top in the SERP, it will just not happen.
SEO is not something that you download and install once and it takes care of your ranking, however, it is an ongoing process that needs consistent effort from your end in order to succeed. You need an SEO expert working with you continuously to help your site rank well in the SERP.
While there are 2 great plugins that can benefits you like “All-in-One SEO pack” as well as the “WordPress SEO” which is released by Yoast. There are several other plugins that are available online, which needs to be properly studied before installation.
There are various factors such as quick installation as well as server management that should be considered before selecting a hosting platform. You need to check to see if the hosting solution provides you plugin support, storage and recovery, popularity etc.
The answer is both positive and negative. If you use themes that are genuine it will not effect, however, if you use themes from a spammy link, it can drag down your ranking big time.