Synthetic oils are a whole different thing than conventional.
Many synthetic oils are engineered in such a way that for multi viscosity ratings, they don't have to add anything to lower the first number. They handle the heat and stress inside the engine much better than their conventional counterparts.
The main reason why a lot of people say not to run them is if you're doing normal driving and normal oil change intervals (the ones recommended for conventional oils) there really isn't much benefit to them.
Some people also complain that seals start leaking after changing to synthetic oil from conventional, which does happen sometimes.
Synthetic oils really shine when you are talking about extreme conditions. Super cold/super hot temperatures, or running the engine at max output for extended periods of time.
They also last much longer than conventional oils, so if you aren't afraid to increase the miles between oil changes then the cost difference is negligible or non-existent in many cases. You do have to pair it with an oil filter designed for extended intervals though for best results.
Problem here is you don't actually know how long you can run the oil before wearing it out (so to speak) the only real way to know is with a used oil analysis.
The analysis costs money. I forget how much, but it's something like $30 or $50. Compare that to the cost of an oil change, and it doesn't make sense. 2 gallons of conventional oil is about $30, and a filter is another $15 or so. You can change the oil for about the price of the analysis, which makes it a moot point.
I will say though I'm running Mobil 1 M1-302A filter rated by the manufacturer for 20k miles between changes, and Triax Fleet Supreme full synthetic 15W40 oil rated for 25k between changes by the manufacturer. I'm about 8,500 miles since I changed the oil in it, and will probably go to 12 or 15k miles before changing. I don't usually work my truck all that hard, but I have towed with it a few times with enough wind resistance that I maxed out my power at 67mph (gen/ecu trailer with tents and the BEOD installed on the truck. I crossed the scales at 13.5k lbs in my M1123).
