removable swan neck ones are fine – you just have to get the right carrier fitting
couple of reasons, just my experience so I could be wrong.
I prefer the rack to be bolted solid to the flange, it just feels more secure to me. Having the towbar fitted is much cheaper. There seems to be a wider range when buying a rack, more options normally means cheaper.
Not having the actual ball fitted wont save much as that’s the cheapest part, I’d have it fitted anyway it may help when you come to sell the car.
I use a company by me called CDF towbars, the last one I had fitted cost about £20 more for them to come out & fit it than it would have done to just buy the hardware & fit it myself.