OK. I might give it a try - in a setup where I will be placing trades in a demo account at one broker, which I am sure will present just ideal, unrealistic fills and trading conditions, and copying the trades into a real account at another broker. If there is any kind of significant difference between the two, I will conclude that the broker holding my real account is not offering a good service, whether intentionally or not.
The main thing is that I can indeed trust one of those huge brokers, like IG, to at least always pay out my money when requested - no doubt about this. What they do in terms of trading conditions, is another thing though, and I will just test it. Perhaps all those negative opinions are unjust. We will see.