Many public speakers are fluent in many disciplines. We often have a number of different presentations on various topics to get the most coverage of an available audience. But while that may seem valuable to a meeting planner or corporate decision maker, having too many various topics on a website can cause trouble with the search engines.
Trying to cram too many topics on a website can make you look like a 'jack-of-all-trades' to Google. Search engines give higher ranking to websites that look like an 'authority' on a single subject, rather than ones that are stuffing too many keywords in their site. We may not be doing this on purpose, public speakers are known for being very diverse. However, it can be hurting your chances for better search positioning.
A website is better suited to optimize a niche. Not only does the jack-of-all-trades thing make you look bad in the search engine's eyes, it's a more powerful strategy to focus on as many various keywords on a single topic than to spread yourself too thin. Think of all the keywords you may not be using to capture traffic because you're using too many 'broad' keywords to fill all the things you teach about.
Sometimes it's better to have two separate websites if your topics are far removed from each other. If you speak about time management and keeping teenagers off of drugs, those should be two different websites. There is no way to cram all the keywords for both of those topics into one website and have it be effective. It may look impressive to your website visitor, but they may never actually find the website in the search results. I know it's more work, but the payoff will be substantial.