IMO- lack of properly located heat or simply lack of heat is the #1 reason solder joints fail.
For over 2 decades I have soldered pipe ranging from 1/2"- 4".
I have been involved in unscientific "what if" tests and have decided that some steps people say are needed to solder don't always apply.
But 4 things are required- Clean material, flux, solder and heat.
The rest is a matter of opinion, preferences and jobsite conditions