Never... I can't believe the Hang were not mistreated to begin with to require retunes! people need to treat them with respect, don't use soft mallets and don't hit them too hard (or drop them), like anything they can go out of tune, and the downside of it is you have to send it back to PANart, unless of course you know how to tune a Hang yourself.... which is more complex than steel pans, people buy and play those but not everyone can retune them themselves either.
I know pleanty of people that busk with their Hang and have never had any problems, remember new they are 1200 euros, mine when i got it cost me £350... eBay is pushing prices up to still unaffordable prices but that's only through the demand, those that buy at the silly prices probably do not need to busk you're right... but some spend, save bloody hard all the money they can just to own one.
Personally, although these steel tongue drums are ok, they're not the Hang and to be honest i'd rather spend my money on a proper wooden tongue drum, the sound and resonance is way superior to that of the steel, wood is way more natural in tone and sweet on the ear. www.tonguedrum.com
I'd imagine with the Milltone drum that you can apply magnets to certain parts of the metal to change the pitch, I would think that if you knocked it out of tune by bending the metal then you'd have to know what to do to bring it back in tune... much like the hang.
There are no alternatives for the Hang at this time, the Halo is not on general release as yet, the BELLart from Spain is out there but the tone is still not quite that of the Hang and the waiting list is long.... 1200 euros too so the same price as the Hang but not yet the quality of the Hang... make your own mind up on that one.
Halo is the best bet for anyone in the US, but $1500 I though it was coming in at less than that to buy... more like $1200? the market dictates the price of these instruments... I do however believe that new makers to the instruments shouldn't have comparable prices with PANart, the experience and the client base just isn't there for them and they could well be setting themselves up for a fall... better in my eyes to start low, build the product and then increase the price.... but that's just my opinion.
Kalimba are nice though... you still have to pay for the quailty there too? right?