Heat pumps are generally very reliable. Just like with boilers, most failures come from the heating system side e.g poor water quality resulting in failing valves and pumps, these things probably wouldn’t be covered under a heat pump warranty anyway.
If the heat pump was to fail, it would usually be a replaceable component, there are not many components in the heat pump, compressor, EEV or 4-port valve. The expensive item would be the compressor (£1k ish), but luckily compressor failure is very rare.
A heat pump is most likely to either fail in the first year, or run for decades.
The cost of the heat pump unit itself is only part of the installation cost, e.g typical 7kW heat pump cost around £2-4k.