Any boiler must have some method of limiting pressure to safe level, and Watt and Trevithick argued about this back in the early 1800's. Watt wanted low pressure, Trevithick used high pressure however the big change was a fuel supply that can be turned off quickly.
So in real terms with gas and oil we can turn off the fuel supply to stop over heating, with solid fuel it is a lot harder, so safe method is 22 mm or larger pipes so when a side or back boiler actually boils the pressure build up is limited, clearly the header tank must be able to take boiling water, the safety is only as good as weakest link, so thermal plastic header tanks should be banned.
It does not matter how many over temperature thermostats are added, open venting has to be safest method. Specally using multi-fuel, if the solar panels are any good, they can over heat, OK using electric solar panels means they can be turned off, but we have seen the results of an immersion heater thermostat failure so still a risk.
To fit a system which can't be expanded to use more than one fuel is some what short sighted so with a house any bigger than 2 up 2 down the idea of a combination boiler is short sighted.
With modern heat exchangers there is no reason why the DHW should not go through the hot coil so you can still have hot water same pressure as direct cold water even with a vented system.
Be it a high pressure boiler in a heritage steam train or a house central heating, it needs boiler inspections at regular intervals, we see homes still with no earth to lights when they should be inspected every 10 years or change of occupant which ever sooner and the rules changed in 1966. So it is clear high pressure system will no be checked, and the lack of inspection hatches to inspect shows how poorly made the systems are.
Even from new many homes have dangerous systems installed, my father-in-law had a gas fire fail, on removal it was clear the flue bricks had never sealed, gas safe guy fitting new sealed it with ducting tape, which peeled off once hot. So clearly can't trust tradesmen to install safely, so best idea would be to ban sealed systems until some testing procedure has been developed.
