In an ideal circumstance mouseovers are insubstantially better than clicking. Unfortunately, ideal circumstances are not realistic. I personally prefer clicking targets rather than mouseovers as it gives me much better feedback on my actions. With a mouseover, I can only assume that it's healing the correct person and I've learned by quite a few deaths that it's not always the case. The only benefit of mouseovers for me is just quality of life. Playing a paladin, monk or priest with mouseovers means that the boss never needs to be untargeted and makes gameplay much more fluid. That's it.
Yes, I do use mouseovers, but for important spells such as Hand of Protection, Pain Supp, etc. I still use clicking for the reasons above. I've never had a death from the sub 100ms delay. Honestly, a person would gain more from changing actions to be on button press rather than button release than they'd get from mouseovers or clicking.
This is like an argument of using Grid/Vuhdo/any raid frame addon vs the default UI. It's personal preference, and the person will perform better with what they prefer even if it is theoretically slightly worse or better.
edit: By clicking I mean clicking targets, obviously I keybind the spells.