My only experience from this line of spells is from
https://wiki.project1999.com/Blazing...r_of_Fennin_Ro
I use it for pulling on my Shadowknight. The long range and delayed hit are amazing for pulling. You can cast it, and get two or three med ticks before you have to stand up to prevent getting max damage from getting hit while sitting.
The range is also beneficial if you want to cast a second spell at maximum range. Pull the mob with this spell, and spam the second spell you want to cast. That way, the second spell will start casting at it's maximum possible range.
As to which mage spells are most efficient in terms of DPM, that I cannot say. I have never played a mage for long.
You are correct about the downsides. It is possible for the bolt to be interrupted en-route. I rarely have a problem with this, however. I have only really had an issue in hilly outdoor areas. If I am at the bottom of a hill, and the mob is at the top, that can be a problem.
You can easily do a few tests in the area you are fighting in before you commit to it. Just use your lowest level spell to save mana.
EDIT: Also be careful when casting this spell on pathing mobs. If they path behind a wall while the bolt is en-route, it can be interrupted as well.