I don't agree with the myth part, it's pretty easy to test yourself by dropping the temp of your boiler, I know what temp my boiler will and will not heat my home at because I've tested it.
Heat Geek and Skill Builder have plenty of videos on the subject, a lot of money over and above the cost of a heat pump is needed to be spent on older properties on insulation, altering the design of the property and introducing new problems
A heat pump requires bigger rads and pipework when compared to a gas boiler (due to the surface area needed) because of the lower flow temperature. You can't just drop your boiler temp and think it is the same. The only myth around them is that heat pumps don't work! Of course they work. Like everything else in life though, it needs to be done properly to work well. If your central heating system has cheapened out and used small rads and microbore pipework, you are going to have a bad time.