Obviously there are things you can deduce easily. Like the larger consumers (but not so large they use abnormally sized and expensive boilers) have more to gain that the smaller users.
I think that there is not a lot to choose between the reliability of modern condensing and non-condensing boilers. If anything the more corrosion resistant materials used in the condensing unit may even give those boiler a longer life.
So the debate comes down to how does an inefficient simple boiler stakc up against a modern boiler. If the modern boielr can pay for its extra maintenance costs in saved gas then a condensing unit will certainly do so.
Also to be noted is that a number of the failures of older systems relate to components that would be housed inside the boiler on modern systems. So failures of the pumps, timers, thermostats are clearly no different.
On combis (slightly to the side of the main debate) we have the water flow detectors, (maybe) diverter valve and secondary exchanger against zone and/or diverter valves, cylinder(s) and cylinder thermostats. That's not clear to me which is the less reliable. Sealed Systems v. open header (including all the problems with badly installed open headers). That comes down to the expansion vessel v. the header tank (overflowing, jamming dry, limescale plug at the circuit entry). I'd say it a comfortable points victory for the sealed system, The cost of fixing all the failures is dominated by labour charges.
The gas valves are clearly comparable (a very reliable component in all systems anyway).
So now we are down to the nitty-gritty: Does the fan, APS, PCB & temperature sensors give more trouble and costs that the thermocouple and the extra gas used.
Let's say that for a typical house the difference is £100 year of gas saved (£400 v. £300). I assert that's a realistic test case. A reasonable repair cost for a the modern boiler is around £200 - (half that if you can diy). So if your modern boiler breaks down less than every other year you should be in, you can afford a less reliable modern boiler if you can diy.
I'm have tried to be fair and balanced here. I'm open to correction by reasoned (and preferably numerical) argument.
I'd say that if you buy middle market or above you should save with a modern boiler. Of course if you happen to buy one of the lemons that have been hawked around in the last 10 years you may lose.
HTH