Fur Coat / Ice Scales: As I see it, the primary niche of these abilities for this project is thus:
it has the unique ability to give defensive utility to unresistant typings without straining the process elsewhere.
At the power level of the average Pokemon, 2HKOs off of neutral hits are fairly common, and, barring Recover or Regenerator, 3HKOs are practically ubiquitous.
It’s easy to forget just how much the ability to stomach neutral hits is dependent on access to extremely powerful recovery. Looking at the OU suite of genuine defensive mons without it, you’re limited to Steel types (

,

,

) or those with a stellar combination of various defensive traits (

,

).
This isn’t only relevant to defensive mons; any non-glass-cannon lives or dies on its access to resistances.

’s dominance is largely driven by having 2 immunities and 6 resistances, alongside two barely resistable STABs.
Weaknesses are negligible by comparison;

, despite having 5 weaknesses, has plenty of defensive utility thanks key resistances to the incredibly common Ground and Water.
This can leave typings without many resistances out in the cold - Ice, Rock, Normal, and Psychic being the most notable (but far from only) examples.
This issue is particularly important for CAP 30. Sharing a type between two defferent but equally-matched mons increases the demand for that type to be well-rounded; otherwise, both forms might be forced to fill the same archetype. This ultimately biases us towards more defensive typings; there are a myriad of ways for an ability to turn a primarily-defensive type offensive, but there really aren’t any that can turn a primarily-offensive type defensive. Fur Scales is the only exception.
Ice Coat is best considered as a supplement to typing, rather than stats. It converts neutralities into resists, and weaknesses into neutralities, but only on either the physical or special side.
While strong, it is far from broken. Even ignoring the physical/special limitation, Ice, Rock, and Grass would still have fewer resistances than Steel; mons such as

prove that having a wide array of resistances isn’t enough to make one an impenetrable wall.
TLDR, it goes without saying that, like any ability, it takes
some work to keep Fur Ice Coat Scales balanced; you can’t just slap it on

and call it a day. But it is perhaps the single
least restrictive way to explore broad archetypes like “defense without recovery”, and places so few demands on type that it can enable offensive typings ability 2 can explore to their fullest.
A Quick Aside on Filter/Solid Rock
As it’s the only other defensive ability being discussed, you may be tempted to compare Filter to Fur Coat, and say something like “it can achieve the same thing”. Do not be fooled; these abilities are nothing alike.
The damage reduction Solid Filter applies to super-effective hits is roughly equivalent to knocking off a Life Orb. While this is nothing to sneeze at, it doesn’t change the mon’s defensive profile that much. You still don’t want to take super-effective hits; at best, it converts some OHKOs into 2HKOs, to allow you to fire off a critical move at a key moment. It doesn’t give you any new opportunities to switch in, and it doesn’t alleviate any pressure from neutral hits, which are some of Scale Coat’s greatest utilities. As I mentioned above with Rillaboom, resistances are the key to defensive utility, not a lack of weaknesses; Filter Rock does nothing to help in this department.