Several factors. I'll note them in the order of importance to me:
1. Significantly better motors. Think classic Mercedes engine vs. Yugo. Also, typically run at higher rpms which lead to better finish and quicker finish times.
2. Motor isolation. If you've had a cheap vibratory tumbler, it sounds like someone is running a jackhammer. I have a Berry's tumbler (a cut above the HF models) and I would have to put it in the far end of the garage with a wood box over it to deaden the sound, otherwise I could hear it ANYWHERE in my 2 story home. I can sit my Ultra-vibe 18 out by itself and close the door to the garage and not a sound. Definitely helps lessen the complaints from the family regarding noise.
3. Much better bowls and lids. HF bowls aren't made for longevity. They warp and crack pretty easily. The lids crack very easily if you over-tighten (which is common because the seal between the lid and bowl is not very tight). Here's how good an Ultra-vibe bowl is. One of them I bought, the previous owner did some wet polishing but didn't take out the media. Over time it became a caked-on mess. Impossible to get off. I put some aggressive media in it and ran it for 8 hours. It came out spotless.
Again, you get what you pay for. You can get the HF models on sale everyday for under $100. The big 4.75 gallon Ultra-Vibe 45 retails at $650. But as I said, if you look and are patient you can get them for far less. As I said in the earlier post, I paid under $100 for TWO Ultra-Vibe 45s and I've seen several since in the $100-200 range a piece for tumblers in excellent condition.