The problem with freewheels is that the axle attaches to the frame at some distance from the drive-side hub bearing. This creates a lever arm that flexes the axle with use and can eventually fatigue it and fail. The more cogs you pile onto the freewheel, the greater that distance becomes, and the more leverage is exerted on the axle. That's why you don't find modern hubs with nine or more cogs using freewheels. Instead, they use a "freehub" design with the ratchet mechanism built into the hub. This allows the drove side bearing to be moved outboard and provide better support for the axle.

And BTW, a solid axle doesn't make much difference. I've seen as many solid axles fail as hollow ones. The good news is that bent or broken axles are not catastrophic. I've known people to ride around for weeks or months with broken or bent axles and only realize it when they have to remove the wheel to e.g. fix a flat and the broken axle falls right out of the hub.