Using WordPress as a full CMS

I’ve been making hand-coded websites for 10 years now. Only recently have I started using WordPress, partially because I’ve never needed it to help me build a website. But I’ve been very impressed with WordPress so far and have now installed it for 5 or 6 clients. I know people use it as a CMS, even without using the actual blog part, but I’m wondering exactly how much that’s really true.

I see that there are plug-ins that allow me to add PHP to my WordPress created pages. I haven’t tried them yet but since I can already edit HTML of a page’s content area, if I can also edit PHP and connect to mySQL databases (unrelated to WordPress), it sounds like I can do just about anything I would normally do by hand-coding a website. Am I wrong about that? Is there something I’m missing, or am I right that if I like a WordPress theme, then there’s not much I can’t accomplish in the content area of any page, right? Are there any other pitfalls I should be aware of?

I’m thinking it may be nice to just log in and change some text and images online (even if I need to use the HTML editor) next time a client makes a quick change, rather than breaking out the raw php files and uploading each to the server using an FTP program.