Topics, no. I can write about anything. There are, however, levels of expertise needed that are beyond me in some areas. I cannot, for example, learn enough about quantum physics to write an original research paper on it. I could (and have!) happily written an article explaining what quantum physics is, what the main theories are, controversies in the field, inexplicable concepts, etc.
Likewise, while I wrote a weekly blog for a law firm for years, I am not a lawyer, and cannot write legal documents, or explain legal systems and laws at the level of understanding of a qualified lawyer.
Medicine is another good example. Many of my clients run health and wellness businesses or holistic practices, everything from pilates to biohacking, yet I am not a doctor; I am not qualified to write at a clinical level.
Law, medicine, engineering, and science are the areas where you will find there’s a limit to the level I can write at.
That being said, I am a technical writer and have a great deal of experience writing for tech companies providing services like CAD and workplace sensors. So, I can write about technical subjects to a degree, provided there are ample specifications and data for me to study to learn about them. The same is true for the health and wellness treatments I’ve written about – I don’t need a degree in medicine to explain how they work, provided there is information available for me to learn how they work.
Research is part and parcel of the job. I’ve researched some bizarre and wonderful things over the years, so much so I’ve become a fountain of esoteric information on a veritable smorgasbord of subjects. If you’re unsure if I will have the research chops needed to write about your subject, just ask. I will always be honest about my current level of knowledge in the area, and confidence level in researching it to the degree you require.
It’s useful to remember that before I was a crackerjack copywriter, I studied archaeology to PhD level. I’m really good at research!