He's definitely mediocre with his feet but if you are playing a defender who is good with them like De Ligt or Bonucci, or have a DM coming short like Pjanic did a lot and Arthur tries to do, it is not quite so important. He's not a sweeper keeper, he's defensive, he has the wrong kind of build to be the former because he's a massive beanpole. When keepers like him get in trouble they just have to kick it out, there is no point trying to lolzilian your way out of it.
He has obvious flaws to his game and is no Buffon or Zoff but it's obvious why we would be interested in the circumstances. I feel sorry for Szczesny as he's, for me, in exactly the same category of 7.5-8/10 keepers who sit below the very top ones, but the kind of mistakes he still makes at 31 years of age show that he will always make them. He seems to drop in confidence, and when that happens the whole defensive unit loses its confidence.
Donnarumma might have played a lot of games and grown up quicker than almost any other keeper in history but he does still have lots of years to improve and learn. Experience is crucial for a goalkeeper, but at some point in their 30s the reflexes and agility declines and that negates what they have improved on in terms of positioning, decision-making and awareness.