I spoke to the technical manager at length, and the best he could come up with is "voltage spikes" and suggested I check with my energy supplier.
even though I knew this was probably BS, I did, they said my power supply is fine.
I am most reluctant to replace with another Bosch unless they discount
Voltage: Possibly an issue but unlikely. Your light bulbs especially would all have blown long before excessive voltage damaged an element. Elements are pretty tolerant to excessive voltage (within reason) unless we're talking about halogen elements which are more sensitive (like a light bulb...).
Assuming we're not talking about halogen elements, if voltage was the issue then you'd have had a lot more problems than just the cooktop unless we're talking about a short, sharp spike that happened when nothing else was turned on. I assume the elements didn't all fail at the same time?
Low, as distinct from high, voltage certainly won't damage an ordinary element in a cooktop (it will destroy your fridge pretty quickly however).
Frequency: Outright nonsense if anyone's claiming that to be a problem assuming you're on the NSW/QLD/Vic/SA main grid. Large frequency variations sufficient to damage ordinary household equipment just don't happen on that system.
Unless you're running appliances via a frequency change device (eg a variable speed drive), NOTHING you do in the house will affect frequency. Running a welder or a big air-conditioner starting up will make the
voltage dip and you'll see flickering lights. But
frequency is always the same from the power station to your power points at home and there's nothing you can do to change that. And for frequency to change in Newcastle, it would have changed everywhere from north Queensland to SA - it's all the same grid.
Frequency would only be an issue if you're running it from a generator, inverter (other than a grid-connected solar inverter which is inherently tied to grid frequency) or are on a very small, unstable grid (eg one run by a mining company primarily to supply their own operations with supply to households being a by-product - frequency may wobble when the mine starts or stops large motors etc).
So I'd say it's almost certainly either faulty design or faulty manufacturing. The simmerstat may be allowing the elements to run too hot and burn out (
note that the simmerstat should fully offset any voltage fluctuations within a fairly wide margin - that's an inherent feature of its design). Another possibiltiy is it's just faulty design leading to exceessive heat build up. And it could just be faulty manufacturing.
One point to note for longevity of cooktops - don't leave them running flat out with no pan on the hot plate. It shouldn't cause immediate damage, but it will be running hotter than it should which isn't good. (Why would you want it running without a pan anyway?)
As for Bosch in general, personally I prefer to stick to very mainstream whitegoods. Easy to get parts (and they're cheap) and the quality is generally reasonable. Once you start putting fancy electronics into everything and using specialised parts you can forget any prospect of economic servicing when it inevitably breaks - parts will cost a fortune and won't be easy to obtain outside the major cities (noting that "major cities" in this context means Sydney and Melbourne only).
In terms of cooking performance, I'd go for a gas cooktop unless there's a good reason not to. Simple, not much to go wrong and they are much better to cook with in my opinion.
For the record, I'm a licensed Electrical Technican (A-grade electrician).