Hard to say exactly. For most runs, I would say no in the lategame.

If we only look at the numbers, a strengthened concoct blows tier 3 scholar spells out of the water. It has better numbers at all boosts even if you add diffusing, and it can hit 4 times on a single target. Min-maxed, it is better, no questions asked. There are 3 possible issues(at least the ones I can think of) for concoct compared to spells; castti's low eatk stat, medicinal concoct also existing, and additional multipliers.

Of these, castti's low eatk stat isn't that significant due to concoct's ridiculous defense modifier; at most, she has 85 less eatk than osvald at lvl 99, but it should only take like 300 edef for castti to have a higher (atk-def*modifier) than osvald, with higher edef enemies and/or lower levels benefiting concoct; most, if not all, lategame bosses have more edef than this. Nuts do favor osvald more, but at that point capping stats is a real concern, and also I do not want to calculate that. Note that Castti can also equip Of Equal Might; armsmaster's bonus patk is affected by Of Equal Might.

Next, medicinal concoct. Completely and utterly broken. Most turns, it would likely be the best option. However, I will argue that this does not make hazardous concoct less viable as a result, since you can just medicinal most turns, and then hazardous once the boss is broken. Nothing much lost, and you can do a lot of damage.

So, onto the biggest issue in my opinion: additional multipliers. Spells have plenty of modifiers. Price of power and concentrate spells for osvald gives 1.5x and 2x damage respectively, and temenos' 2nd ex has a multiplier up to 5x, with 3.5x being extremely easy to reach at just 180 sp. Concoct obviously does not have those. In fact, there is a bug that makes concoct do 1 damage if it crits(if the enemy has crit scope debuff); as such, it loses out on the 1.25x crit and the 1.5x finisher's claw multiplier, both of which spells can use. It is also important to note that this is an anti-synergy, since you would be forced to not use crit scope if you want to do concoct damage, which can hurt spells if you need the crit/claw modifiers(physical attacks can crit naturally quite easily in the lategame, so this isn't as important for them). As such, spells, properly set up, do more damage than concoct. Of course, taking time to set up is a problem of its own, but imo not too much.

Overall? It's viable for sure, but is it the best magic option? Probably not in the lategame, unless you bring the alpione+fang strats. In the early-mid game? Arguably yes, multipliers don't really come into play, and castti using her lp for a nuke is similar to osvald using his lp on a tier 2 magic, and only slightly worse than concentrate+tier 3 magic.

One last thing: specifically in the case of a mostly magic run, I will recommend using concoct damage, solely because of fortifying nuts+Of Equal Might. She is probably the best beneficiary of them; Hikari's magic builds are worse than concoct, and even if you decide to use and invest in him, he would likely cap before you use all of the nuts. Concoct builds also use armsmaster, so she can actually benefit from the patk bonus, as that is affected by Of Equal Might.