Join Date: Dec 2009
Location: Dallas-Fort Worth Area
A character on a monster cannot join a unit (of non-monsters) as explicitly stated in the BRB (too damagerous to the other models in the unit), but the Jugg is not consider a "monster". A Jugg is a "monstrous beast" (such as a Daemonic mount, Rat Ogre mount, or Jugg) and the character on the mount is considered "monstrous cavalry" which, confusing enough, is not the same as a character on a monstrous mount (such as a stegadon or dragon). A character on a Jugg or Daemonic mount, therefore, can join a unit, BUT it doesn't get a Look Out Sir! unless it is in a unit of five or more rank-and-file models (including musicians and standard bearers but not champs) that are also classified as monstrous cavalry. Thus, the only character on a Jugg able to get a Look Out Sir benefit would be a Herald of Khorne on a Jugg in a unit of 5 or more (non-champ) bloodcrushers in a Daemons of Chaos army.
As explained on p. 99 of BRB, the character on a Jugg in a unit cannot be targetted in a unit by "normal" shooting (shooting based on BS) but can be hit after one shot has been allocated to each other model in the unit. However, the character can be targetted by certain template and line shooting and missiles that are not allocated across the unit and gets no Look Out Sir! at all from shooting or ranged magic attacks. That means that cannons, stone throwers, mortars, warp-lightning cannons, flame templates, and magic spells that use templates or can target directly will hit the character (if a hit is rolled) and the character will get not Look Out Sir! roll at all.
The meta game is shifing such, with no guess range in 8th edition, one will start seeing more war machine shooting such as warp-lightning cannons (skaven), cannons (empire, dwarves), and stone thrower-type weapons (brets-trebs, orcs & goblins, dwarves-grudge, empire-mortar, and Tomb Kings-skulls) with high S and multiple wounds upon direct hits that will kill characters (absent a successful ward save) within a turn or two on average, especially if accompanied by re-rolls of misfire or scatter dice in some of the armies. Additionally, there are quite a few template and direct damage magic attacks that will hit a character and potentially kill it absent a Look Out Sir! but could, otherwise, be avoided with a successful look-out sir roll. Thus, putting a character on a mount in an infantry unit or putting a character on a Jugg in a warriors of chaos army will expose the character to much greater risk of being killed with a lucky shot or spell from certain war machines and certain spells that, otherwise, the character could avoid with a Look Out Sir! roll if the character were in a unit of five or more rank-and-file models of the same troop type as the character.