But due to the nature of the FE:LH only time there's going to be a useful nature cloak enchantment up is in a single faction that managed to control enough earth shards and anyone else is very likely to not have enough earth shards to resist a blizzard via the earth's cloak enchantment.
I don't know that I agree with this. Assuming that you have a unit with the proper gear (meaning one of the cold-resistance cloaks) you should easily be able to get cold resistance into the 50-70% range. What it lacks is an easy mass-casting option, but that's where Protection from Cold for a Mage with the Abjurer trait comes in - +50 cold resistance on all allied units means that with one spell and the right cloak most of your army could be immune to Blizzard (unless you're playing Quendar). Less than total immunity isn't going to help low-level troops against a decent Blizzard caster, but it can help the higher-level ones survive long enough to actually do something about the enemy army or caster. Also, in all honesty, I don't care if some of my low-level units die to Blizzards - they're there to tie up enemies so that my high-level casters and backline archers can shred the enemy army, and as long as enough of my front line survives to tie the enemy up I really don't care what happens to the rest of them.
Beyond that, I tend to feel that Nature's Cloak is not a good go-to answer for general elemental resistance - casting it on enough units to matter, whether against AOE spells or single-target spells, costs too much mana to maintain on the strategic level for groups of units, and saves you at best two turns (Protection from Fire + Protection from Cold) and maybe 30 mana at initial casting (assuming you cast both protection spells, which strikes me as typically unnecessary for a single battle), so unless you really need the lightning resistance or didn't include a decent caster in your army, there are much better options available for dealing with high-end elemental damage. Mind you, I tend to feel that the best way to 'protect' your army from elemental damage is to make killing the enemy casters the top priority for friendly archers and casters, rather than spending a level to get the Abjurer trait or blowing mana on giving Nature's Cloak to a bunch of random units. On the other hand, Nature's Cloak is reasonably appropriate for rounding out a champion, if you're willing to pay the upkeep or recast it every now and then, since it's hard to find lightning resistance and Nature's Cloak rounds out the resistances given by Ice Warg or Dragon Hide Cloaks rather nicely even without any Earth Shards (and with three or more shards and an appropriate cloak, you can become immune to either Fire or Cold damage).
I'ts only when spell resist reaches 100+ and spell mastery fails to surpass the spell resist that only then spells become useless and actual unit design with spell resistant trait becomes very useful.
If a regular unit's spell resistance ever exceeds your caster's spell mastery, then your caster was badly out-leveled since you start out with something like 50 more spell mastery than spell resistance, unless you're dealing with Ironeers, and your caster should be gaining experience at twice the rate of regular troops, or unless the unit design really stacked up on the spell resistance and trained out of a Tower of the Magi Conclave. Additionally, the spell resistance trait is decently useful against mid-level secondary offensive spellcasters - meaning non-Mage champions with decent curses or damage spells, or Mage champions built for summoning/healing rather than something more direct (since Mage champions will usually have at least one decent offensive spell almost regardless of how they're built, but if focused on summoning and healing might not have invested in the Prodigy traits yet). It's less useful against damage spells than against curses since most of the decent damage spells tend towards 'resist for half damage' rather than 'resist to negate', whereas all the curses are 'resist to negate', and even at half damage the decent damage spells are still worth hitting with at mid level or higher (10+ or so, though using Flame Dart may not be worthwhile without at least Warlock or Evoker I if you don't have a decent chance to bypass resistance even into the mid-teen levels, though why you'd set up a caster with Fire as a non-Evoker Mage escapes me, unless you just like summoning - but if so, why use Flame Dart?).
I don't know, with the current way the AI uses magic, that I'd design a unit with the Spell Resistance trait, or stack it in unit design, but it's certainly not completely useless. Plus, to continue the elemental resistance stuff earlier - it's fairly easy to get 50% resistance to one element on any given unit. If you can then make it so that half the time you're also only taking half the basic spell damage because you resisted the spell, well, even Blizzard doesn't look so hot at only 25% of its listed damage. Of course, then you have the issue of which element is the right element to choose, and unit-upgrade can screw you with regards to getting the elemental resistance you actually wanted.
