What Makes a Targeted Ability Healthy or Unhealthy?

Riot is on a spree of turning targeted abilities into skillshots. Veigar's update turns his Q into a skillshot; the most recent Ryze rework update hinted at turning his Q into a skillshot; the Sion rework a couple months back removed his targeted CC and gave him multiple skillshot CCs.

On one hand, people are complaining that this is yet another example of mobility creep. Since champions with speed steroids and dashes will have an easier time avoiding all these new skillshots, every change leaves these champions weaker against their mobile opponents, and makes mobile champions in general stronger in the meta than they already are.

On the other hand, Riot's changes aren't coming out of the blue, they are a reaction to a lot of complaints from the player base. Before the Ryze nerfs took him from top tier to bottom tier, there were tons of people complaining about his unavoidable damage, and how it allowed him to bully melee champs out of top lane. And I don't think anybody was defending Sion's unstoppable stun when the rework came around. People hate feeling like they didn't have the opportunity to outplay their opponent, and many targeted abilities give people that feeling.

So what exactly makes a targeted ability healthy or unhealthy for the game? What is the role of targeted abilities in modern day LoL? I have my own opinions, but I want to hear the community's thoughts. I'd also really love for a Rioter to stop by and give us all some insight on the philosophy behind these changes.