I'm not seeing how soul of stone avoids the template and all models within contact of caster getting hit miscasts reliably. It only cuts the risk fo the worst case (2-4) Dimensional Cascade) in half but that just pushes the consequence into the Calamatous Detonation category which is a small template and loss of D6 PD. Pushing a 6 to 7 in a full TG unit is often effectively the same as the template hit, so a 6 is effectively the same as a 7 on the miscast table with in slann in a TG unit, given the size and shape of the Slann model. Depending on the situation, there are times when one might use soul of stone to push an 8 down to 7 and accept the loss of some TG with the slann over risking losing a skink priest with only one wound remaining [or with two wounds (and a cube or scroll) but used for casting the spell (will take one S6 hit from Magical Feedback and one S3 hit from Arcane Vassal miscast rule)].
Similarly, only one of the 5 miscast consequences (10-12 power drain) does not involve losing D6 of the remaining PD. but that consequence means that the caster cannot cast anymore in that phase and loses D3 spells and levels. Early in the game, I would not push 9 (magical feedback) to 10 (Power Drain) to avoid losing the D6 power dice because I often don't want to lose the slann's levels, lose the slann ability to attempt another cast in that phase (assuming the D6 is low or one has a lot of PD left) or lose the spells early and some magic levels off the slann. especially if my skink priest has burned its scroll or cube already or still has two wounds remaining on it.
Fiery convocation is expensive to cast but the consequences for common larger small wound model horde units that do not have high MR is devastating. Usually, it gets off only if you roll IF or they roll badly on the dispel attempt or you roll very high and they don't have a scroll. It often forces people to save their DD or save their scroll and let other stuff through like soul quench. Worse yet for the opponent, the consequences are so severe that, if it gets off in that phase, the opponent will either have to burn a scroll or use a lot of their power dice in the next phase phase just to get rid of it. Against many elite single wound model units with more limited armor saves [naked witches without a cauldron, executioners, ghouls, etc.] the consequences are typically a lot worse than dwellers and definitiely worse than final tranmutation, especially when house rules allow a look out sir for characters in such units. T3 means 2/3's of models are wounded. With a "standard" 5+ armour save, that means 55.55% of models are wounded even if they have S4 (which some elite infantry have). With a 4+ armour save even, the unit (including warriors of chaos without shields) will suffer 44.44% wound rates. An more elite infantry unit with S4 and T4, suffers a 1/3 wound rate to dwellers and final transumtion, but suffers a 50% ound rate to fiery convocation with 6+ AS, 41.67% wound rate with 5+ AS and still suffers a 1/3 wiound rate with a 4+ AS. Plus, the spell is RIP, which forces the oppoent to deal with it in the next magic phase.
Against High Elves, the common BOTWD on white lions hordes in competitive environments means that the spell is usually useful most of the time in that the army against a large spearmen unit, a large seaguard unit, or a number of 15+ archer units in core which may be targeted. I'd roll it against phoenix guard with only a 4= ward if they are a lot of models in the unit as well. The other issue against high elves is the common use of Book of Hoeth to re-roll one dispel dice each dispell attempt boosts the reliability and average roll of dispel attempts makes the spell a lot harder to get off unless the opponent has fewer dispel dice than you plan to cast with. With the miscast risk, you really need the target to average more than 100 points in damage if the spell goes off. Against a unit of archers, that means 15 models or more and no ward save; a similar number (14 or more without shields and 15 with shields) for a seaguard unit, a unit of more than 20 spearmen, a unit of more than 14 swordmasters or white lions without BOTWD.
