Can one website speak to several niches, such as HVAC, plumbing and roofing?

Yes, within one broad category. Home services businesses (HVAC, plumbers, roofers, painters, flooring, kitchens and baths, construction) all want more booked service appointments, so one site can speak to all of them with a message such as booking service appointments for your home services business. Focus on the shared outcome, show the sub-niches you serve with an image for each, and give examples of the businesses you work with. You can tell the AI assistant what you want, and if you do not like the result, tell it to redo it.

Once you cross into different industries, for example home services plus dental plus real estate, one message no longer works because you cannot talk to everyone at once. Putting each niche on its own page or subdomain of one site does not work well either, because visitors who land on the site and think it is not for them leave instead of hunting for their page. Use a separate site for each industry instead.