Why play Veigar vs. Syndra

Syndra has a long range stun that's better to catch single targets, while Veigar has a huge AoE cage that's good to disrupt the whole enemy team.

Usually you want to pick Veigar over Syndra when the enemies have many immobile champions that have to get close so you can easily cage them while Syndra is better if you need that long range stun.

Both have a very good single target nuke and decent AoE damage. It's mainly their stuns that makes the decision. You have to check the enemy team and consider if Veigars or Syndras stun would be better suited.

Syndra is dangerous early and mid game.

Veigar deletes tanks (and everything else) late game. His cage is not only massively disruptive, but can easily catch out enemies who stray SLIGHTLY too close to your team, leading to easy victories and objectives.

Veigar Pros

-Insane unmatched burst -can not be approached by anyone if they are at 65 percent health at that point W/R WILL be enough -Unmatched scaling -one of the best lock abilities in the game

Veigar Cons

-Due to long cooldowns on all four abilities, he is a terrible duelist, he WILL lose a long term fight, it's all or nothing with his combo -Easily lane bullied -Easily gankable -Heavily item dependent

Veigar is better.

Event Horizon (if you master the cast range) is the most powerful zoning/CC/catch ability in the whole game. It can trap/mind-f a whole team.

Plus at 2+ items and with appropriate Q stacks you can carry games in the style of Vladimir/Viktor.

Once you learn how to cast E to catch people on the edge without indicators, you can apply that skill to turn tables on enemies and carry games.

Literally play safe and use objectives/team as bait as you know where the enemy should be when they go in on them.

Also...

Veigar will always scale

That reason is reason enough.