fmmc -- good post and my experience is similar to yours. I have been running UFH for 8 years and spent a lot of time experimenting with its characteristics. I think there is probably going to be a lot of variance in people's experience because of the different ways that UFH can be installed. Mine is laid in screed on top of a heavy concrete slab with foam and foil-backed insulation underneath. I think the absolutely most important thing to realise is that the reaction time is INCREDIBLY slow -- on the order of many hours. When mine is up to temp, I find that I can change the temperature by about one degree per hour. But when it is completely cold, you could easily run it for 8 to 12 hours before the first barely noticeable heat is emitted.
So, I would say the very first thing to do is to get yourself several thermometers and measure the reaction time of your UFH in different zones. And don't rely on a single temperature reading per room -- I find that on a cold night there is easily a four degree difference between the centre of a room and a spot below a window. There is also a substantial difference in reaction time between tiled floors and wood or carpeted floors. Wood floors are more insulating and need a corresponding higher temperature gradient to push the heat through. In my case I set the thermostat a couple of degrees higher in a room with wood floors compared to tiles.
I regularly spend long periods out of the house. If I know I'm going to be leaving for a while, I would turn the heating off at least 12 hours before. In the same way that it takes that long to heat up, it's the same when cooling down. It's important to measure how fast the temperature changes, both up and down. That will give you an idea when you want your boiler on, or setback timer set, before the morning/evening periods when you want higher temps. As I said, my experience is that I can change the temp by one degree per hour. It would be futile to think I could change from 15 to 20 C by having the heating on for two hours before 8am. You HAVE to experiment with your own system though.
Regarding boiler capacity and the suitability of oil to match UFH, I'm not sure what the fuss is about. The temperature of my boiler water bears no relationship whatsoever to the UFH temps. They are connected by an inlet valve and mixer valve. When the UFH needs warm water it opens the inlet valve. It doesn't matter what temperature the boiler water is at -- the UFH will just let in more or less, to achieve it's own desired temp by mixing with its cold water returns. You can set the temp of water going to the UFH ... up to a limit you can improve the response time by increasing the water temp at the inlet valve, however there is a max recommended temp and there is an automatic cutoff if it is exceeded. A problem I had for a long time was an intermittently faulty inlet valve which stuck in the open position, so the heating would appear to work for a couple of hours and then the auto cutoff would kick in and nothing else would happen. (There is also a balancing valve at the other end of your manifold, similar in function to the shield valve on a radiator, but I wouldn't go near this if you don't have to).
Someone mentioned burned out thermostats. I think you probably mean burned out actuator heads on the UFH manifold. These are the little devices sitting on the returns of your UFH circuits that open and close the individual circuit valves in response to your zone thermostats. For some reason they do seem to fail from time to time. If you pull the head off (it's a little squeezy spring clip) the circuit will be permanently open. If you want it closed there is a little square nut that you can screw down. It's easier to see what's happening if you've got flow meters on your circuits -- a transparent perspex tube with a tiny sprung plunger that shows you if water if flowing -- but not all UFH manifolds have them. I got replacement actuator heads by ordering online from a company in Germany, and I now keep several spares. I think I've had two fail in eight years.
(It's a long story, but when my house was built the heating was only half installed because of a dispute between the builder and heating subcontractor, which I only found out about when nothing worked in the first winter. Both the people who installed it and the distributor who supplied the system refused to have anything to do with sorting it out, so I've been learning how it works and maintaining it myself ever since. I can tell you from bitter experience that dealing with an east German company through Google Translate was an infinitely better and cheaper experience than any of the gangsters involved in the Irish end).
Anyway, get to know your system. It's not that complicated at the end of the day. You will definitely save money by choosing the correct temperatures for each of your zones, and picking the right times to supply extra heating.