I have played Veigar a fair bit in the past, and would consider him one of my go to mages.
That said I did try out the changes versus the old style, and I have to agree that at best he feels different.
I found that Event Horizon is definitely less reliable, and with the shape of the ability, its a more difficult to plan on actually stunning people right away, whereas "trapping" people tends to be more-so the effect if not "missed. I think some fine tuning is needed to make this ability work as intended, as I think with the added delay, there's some unintended difficulties being added (like not stunning people when it should).
I have mixed feelings about the Q, mostly because if any of the skills are "different" that this would be the highest qualifier. I like the idea of hitting two targets and it feels good to pull it off, and even though in ways I miss the reliability of the single target, I think this change is the most capable of being adapted to.
Dark Matter feels pretty much the same, with the only real major difference being obvious it's combination with Event Horizon, which I'm okay-ish with, so long as Event Horizon works as intended, hence the potential to also be adaptable.
Since really the only change to the ultimate was a nerf to the damage, I can kinda feel it, but a numbers game like that is a matter of more qualified balance.
So, overall I think it's possible to adapt to the changes, and it will definitely be necessary to get anywhere near the amount of success prior, but I also think some fine tuning, particularly with Event Horizon will be necessary to make it feel complete and working as intended. That way should something still feel gimped, it's less of a debacle to make it right.