The big problem I see is that the warranties don't necessarily cover the payback period.
Our case is probably a bit extreme, but currently using 2,000 KWh per annum heating a 180L mains pressure cylinder for 3 people. Hot water is on ripple control (at least 20 hours heating per day, so we don't run out) at 17c/KWh. Rest of house is on an EK MoveMaster style plan averaging 21c/KWh (or 23c/KWh if you only include 'awake' hours).
Looking at dasimpsonsrule's spreadsheet - it looks like at least a $2000 increment for HPHW over an equivalent simple mains pressure cylinder. Also shows average COP of 3.6.
Payback period for a $2k HPHW upgrade (i.e. assuming you had to replace your cylinder, but chose to spend an extra $2k to make it HPHW) varies from 7-9 years, depending on what power price is assumed. I suspect a HPHW wouldn't like being turned off randomly (being connected to ripple control like an old school cylinder), so you end up comparing an old cylinder running on 17c/KWh ripple control power with a HPHW running on 21c power, or 23c when you factor in that it's less likely to be heating overnight.
Maybe you can program your cylinder to do it's once-per-day legionnaires heating cycle overnight - but then again might be cheaper to do that cycle in the middle of the day when the ambient temperature is higher so the COP is higher.
This paper from BRANZ (albeit 10 years old) measured HWHP in service in Auckland and Wellington, finding COPs were in the 1.5 to 2.0 range, unless an extremely low or high hot water use household. I'm sure technology has improved a bit since then, but heat pumps haven't fundamentally changed in that period, so I'd be skeptical about achieving a 3+ COP in the real world. A 2.0 COP could take the payback period to 18 years, and I think counting on a heat pump to perform for 18 years without failing or servicing is closer to gambling than investing, especially once cost of borrowing money is factored in.