I know of no survey. Ask an engineer and he may only tell you what he is familiar with. Also many of them will say a certain combi is unreliable as they are always fixing them, unaware that the model is a best seller, so naturally there are more of them around so more break down. Percentage of breakdowns and number in operation are what counts.
Worcester had a good reputation, that they lost, but are regaining. Their newish Junior model is simple and reliable so far. Vaillant boilers have a "decent" reputation, but suffer from very expensive parts. Potterton & Baxi have a bad reputation, ask Maxie about their poor pcb boards.
Keston had a good reputation but are loosing it as the new Celsius 25 is apparently troublesome. It may be only early model teething problems. Glow Worm a have a good reputation on their new design condensing models.
I have an Ideal condenser heating boiler and an Ariston Microgenus combi. The Microgenus came with a 2 yr guarantee and is well specced. The flue from the outside is noisy, but fine inside.
The Ideal ICOS combi, or is it the ISAR, is now reliable enough after early teething troubles.
The simplest combi's do not have 3-ways valves making them much simpler in design, fewer components and reliability. These are the Worcester-Bosch Junior, Ferroli Modena and Ideal Response. I would seer clear of the Response.