While there is no reason this can’t be used as a primary fermenter, keep in mind the headspace. You would have to ferment less than 4.5 gallons if used as a primary. Beware of blow off too.
I am using a keg with about 1 ½” of the dip tube was cut off. This is a good idea to leave behind any of the fallen yeast or tub. Since I am using this as a secondary, I will throw in a fining agent like gelatin or sparkolloid before I am ready to transfer to clear the beer. With a shorter dip tube, I don’t need to worry about sucking that stuff off the bottom of the fermentation keg and into the serving keg.
Fermenting in a keg has benefits. It is completely dark inside and has a pretty air tight seal. Once the fermentation is done, the beer is easily transferred from keg to keg in a closed system flushed with CO2. There is less chance of oxidation transferring in this closed system.
Since corny kegs are metal, there is no scratching like in plastic buckets or breaking like in glass carboys.