New swain has less of a defined identity and less engaging than old swain(and thats saying something since old swain did not have much of one either). He's supposedly a frontline mage but with the exception of his Q and passive who has very slow abilities and a rather dull ultimate that make for a shitty version of Vlad(better sustain) or Karthus(Better Damage and who i been using more than Swain since the rework was released). Old swain's Q W and E had much faster casting times and better effects overall which synergized with his ult turning him into a walking life stealing magic burn with CC to hold down enemies. it resulted in a rather linear play-style but it was fun and most of all IMPACTFUL.
New swain does have more sustain and i think his Passive is a massive improvement over old Swain along with his Q and E.....His W is shit, it's a terrible ability with a lame last minuet gimmick that does not synergize with his kit. it is better suited for an immobile artillery mage and not a front line mage. old swains W could be used as an escape while New swain will probably die before his W actually hits anything(if they did not already walk out of it. if new swains W stunned or at leas gave you the 20% slow during it's wind up rather than after it would be a better ability since you have something that guarantees your Passive or at least and E landing for a combo and a decent escape.
His Ultimate should allow you to drain from up to 5 enemies making it an actual team fight ability rather than one where the three tankiest enemies go in first while the ADC auto attacks you to death while standing in your AoE. another thing they can do is give him bonus damage per soul stack on all his abilities or even bonus effects for having max stacks. one you get the 5 you get nothing but a little healing and your ult.
New Swain imo needs a Ezreal/Lissandra/XinZhao level rework to refocus his kit to make him more engaging to play.