What Makes a Book Writing Company Truly Professional

I’ve been exploring options to get help with writing my nonfiction book, and I keep wondering—what really makes a book writing company professional? Is it just about delivering clean content, or should they be offering more, like research, interviews, voice matching, and even publishing guidance? I’ve come across companies that seem great on the surface, but I’m unsure how to separate the truly experienced ones from those just outsourcing everything. If anyone has worked with a book writing company before, what were the signs that told you they were the real deal? Any red flags to avoid? Would love to hear from others who’ve been down this road.