The combo I usually see for the full-plate + shield spell shenanigans is fighter 1/wizard(abjuration) X (although fighter 2 get action surge, which is also yummy). Weapon is drawn as last resort, usually to cast booming blade or something.War wizard can already boost saves by +4 with a reaction and not using a spell slot and a diviner can pick a roll. A fighter with indomitable can likewise reroll a failed save and we have an entire thread on here dedicated to how underpowered that is. A Rune Knight can give someone advantage or disadvantage as a reaction every turn for an entire minute.
Also a reroll is not the same as a +5. Advantage is loosely correlated to a +5, but that is only really true when the DC is 10 and only when you have not rolled the first dice. A reroll, when you have already failed, is equivalent to a straight up saving throw with no bonus. When that Green Dragon breaths on you and you need a natural 18 to succeed - If you fail and then use silivery barbs you have an 85% chance of failing again and wasting your spell slot.
In my experience, most of the OMG BROKEN! spells turned out ok 95% of times in actual play. As a DM, I fear Tiny Hut a lot more than healing spirits or silvery barb…This is a great “white room” spell. In actual play? There’s a lot to consider. I think this is why it became so popular as a topic
IF you don't draw a weapon then you aren't doing much damage. Using cantrips maybe. If you are casting real spells then you will not have an abundance of slots for shield.The combo I usually see for the full-plate + shield spell shenanigans is fighter 1/wizard(abjuration) X (although fighter 2 get action surge, which is also yummy). Weapon is drawn as last resort, usually to cast booming blade or something.
If the success chance was 50/50, Barbs is as good as having had a +5 defence; but unlike shield you have to pay for (some) failures. If the success chance was 25%, having Barbs around makes it a 6% chance; about a +4 defence. But 1/4 uses are wasted (compared to a reaction +4). If the success chance was 75%, with Barbs it is 56%; again a +4. Here, however, 3/4 of uses are wasted.
I agree on the crits, not on the legendaries. Most monsters with legendary resistances usually also have high saves, making this less useful when the monster has already suceeded.The real beauty is two fold.
1. You can use it to protect others, including from crits. Crits are 5% chance, so the reroll makes it really really rare. Making it a miss is a bonus.
2. Eating legendary resists. The cost of legendary resists is action economy. Even with LR trumping it, getting an extra chance to force a LR per round is big.
Honestly, I can imagine this ending up being declared a trap spell. You're using a slot, a known or prepared spell, and your reaction for a 60% chance (or so) to change an outcome in your favor. I think they added the granting of advantage to the spell because otherwise no one would ever take or cast it. Even the 95% chance to negate a crit is only situationally useful and the cost is still very high. If I can get my spellcasters to use this instead of counterspell in a round, I'm thrilled.
But it's not +5, it's a reroll. I believe it works out to an average bonus of +3-+5 on average, but unlike shield there is A LOT less certainty that it will work.
Is it good for clutch situations? Yes, I think it would be. But it's enough uncertainty and enough of a resource cost that I don't think it's overpowered as is.
It’s a wizard build, with everything a wizard can do offensively and defensively, albeit one that has traded one wizard level for AC 21 (as soon as they can afford full plate) with spikes at AC 26 (baring magical items and buffs) throughout mid levels when that kind of AC counts the most. If you take your first level as fighter, you get proficiency with CON save at the price of WIS saves, with Warcaster as your variant human feat. Your STR takes the score you would have otherwise put in DEX on a « regular » build.IF you don't draw a weapon then you aren't doing much damage. Using cantrips maybe. If you are casting real spells then you will not have an abundance of slots for shield.
Also this is MAD as you need a high strength, which will largely be wasted since you are not attacking with it.