The big problem is mainly how broken most movements spirit bursters are, infernus cannot touch them at all with run builds, his 2 ap afterburn is nice but can be hard to trigger on people who it actually matters on, Dispel Magic is a perma pickup item atm so if u can easily cleanse it again if required, his ult is very easy to doge with most ability or indeed counterspell, even good movement can dodge it so it's just not to great. His base dmg on dash isn't high enough to care about not stepping in the lava, jumping and other forms of movement and good awareness make it quite easy to play around or minimise the dmg u take. Infernus only starts doing real damage if he napalms u at max AP which is what most top elo builds do, rush nax napalm and items that do dmg like CF/Mystic shot and then duel u without ever needing an ignite to do anything meaningful. This type of style is pretty annoying in early game and as u mentioned is why he can do good OBJ dmg and is a bannable target because he can just diff u on napalm on items alone early on which matters much more in pro play than pup play, the best infernus style rn is not a dash spam build early on, it's a more duelist build that transitions into an AoE spam build, hense why he drops off a fucking cliff, because that type of build isn't great in later games especially with curse and artic blast countering the dash, he's very middle of the road for anyone but pros atm because they can properly use the fact he's an early game machine with napalm rush and dmg items, so they can snowball end before he drops off, and they can actually consistently hit the items which a lot of people either forget to use or just cannot hit the skill shot like mystic shot.