An additional and often overlooked bonus of this form of casting is that IF (and you don't have to) use a Iridescent Horror (the champion) as the focus they channel through, then the shots become 'Precision Shots'
If the unit contains an Iridescent Horror, then he must
be used as the focal point for the unit's Ld test and resolving any Perils of the Warp.
For his cost he's good, but he can also cop it quite easily too.
Basically, if you're going to pull the LOS shenanigans, don't take an Iridescent Horror!
If you're going to simply plunk the unit in cover, he's a good buy and you can simply use positioning to help keep him alive without relying entirely on Look out Sir! rolls.
Heralds are frequently put with them with at least level 2 so you can cast 'Prescience' from the Divination tree (as your first spell has to be from the Tzeentch pool) so you can reroll your misses.
This is 100% not true actually. Unlike Chaos Marine Sorcerers, we have no stipulation forcing us to always take at least 1 power from the respective God's spell lore.
A Herald of Tzeentch can choose to roll ALL of it's spells from the Divination discipline, and/or can include up to 50% (round fractions up) of it's spells from the Tzeentch lore.
Normally a shooty support Tzherald runs Flickering Fire (the Tzeentch Primaris power) + 2 rolls on Divination - one of which is always swapped out for Prescience. (because re-rolls are godly!)
Another important note to keep in mind is that due to the Brotherhood of Psykers rule limiting Horrors to Mastery Lv1, they can never make use of Infernal Gateway due to it being a 'Warp Charge 2' spell.
If you roll this power, you must re-roll it! This is somewhat helpful though for small squads, (such as those generated by the #12 result on the Warpstorm table or through the Portalglyph), as it means you have a 50/50 shot at landing the tank/heavy infantry busting Bolt of Tzeentch power! (which is preferable on smaller squads due to how Warpflame works.