It's because I was involved with a large volunteer-based project that was a literal 24/7/365 operation for several years (dozens of volunteers at any given time and tens of thousands of concurrent users) and can speak first hand as to the differences.

I didn't say it's not worth doing but it will bring challenges that wouldn't exist with employees. Paying people adds a strong motivator to keep toxic behaviour at bay.

Your experiences will heavily depend on the type of project you're running but regardless, you can't hold volunteers, especially online, to the same expectations or standards as employees. The amount of time and effort they can invest will wax and wane and there's nothing you can do about it. Anonymity and lack of repercussions will eventually lead to drama or power struggles when a volunteer steps out of line in a way that they wouldn't in paid employment. There is no fix that'll stop occasional turbulence, it's just the way it is. Not all of your volunteers will be there for the greater good of your community.

Again, that is absolutely not to say that it can't be worth the effort but if you go into it eyes open, you'll have a much better time and be able to do a better job at heading off problems.

I've seen other people express similar opinions to yours and it wasn't until they experienced being in the driver's seat that they understood how difficult it is.

My argument is that it stops being a community when it becomes a business.