And here we go, the feedback on Insanity Mode: Insanity Mode is basically a game of sweeps.
Starting from Florinia and all the way to Ciel, the player's goal is just simply find a way to sweep. But prior to Shelly, there's really no choice but to buy X-Items to assist you, and they're expensive, forcing players to grind for money using Meowth. This is a huge reason why Torchic is among the few viable starters for Insanity Mode - Speed Boost and Bulk Up saves you the money needed for X-Items.
Level Limiter is a must.
Not using Level Limiter makes sweeping RNG based, counting on whether your Pokemon obeys you this turn or not. Sure, you can prevent disobedience from ever showing by putting your sweeping Pokemon 5 levels lower than the cap or so, but this only makes it harder and sometimes outright impossible (ie. Solaris 1) to set up. With that said, Level Limiter is still a double-edged sword, forcing you to use Rare Candies to evolve your starters to their final form before Shelly.
The Early Game Bosses decide over 80% of a starter's viability.
The early game bosses are the worst as the resources your have are scarce. For example, Aster & Eclipse #1 lead with Level 25 Magmar and Electabuzz, a combination that obliterates Fern's team and almost everything in general, making those who picked a Grass starter rely on either the AI screwing up or trying to weaken them as much as possible with Baby-Doll Eyes.
Julia, Florinia, Taka & ZEL #1 and Corey are the other major bosses that hit the similar effects, especially the third one, which I'll elaborate right now.
Double/Tag Battles are especially difficult.
Double battles severely impede the player's ability to sweep, especially 1v2 battles (Although there are only two so far). The outcome of Tag Battles mostly hinges on whether your partner decides to be competent or not. Sometimes, they might not even be able to put in any sort of work. For example, Fern.
Luckily partners receive the 20% extra level boost, so they may be able to carry the entire fight for you sometimes.
Trick Room dominates outside of speed sweeping.
With the level boosts your opponents get, most of the time they're gonna outspeed you, even if it's just that Cradily of Florinia's. Using Trick Room mitigates half of the problem and is in fact very much viable in doubles.
The opponent's lead decides the match.
Instead of explaining why, I'll just lay down two examples and compare them together. Taka lead with Klefki starting from the fight against him on Azurine Island. The AI isn't smart enough to switch out Klefki once it has done its job, so it's easily set up upon. This makes every Taka battle trivial.
Bennett lead with Volcarona in the second fight against him. Volcarona, capable of decimating all Quiver Dancers and Blaziken, forcing me to grind a Blissey and brutally stall its PPs out.
Grinding Post-Agate is ridiculous.
With Clown Indra getting the 20% boost, his team can wipe the floor with you. The best grinding spot post-Agate is when Aya's tagging you on Route 3, and that's only temporary too.
How can everything be balanced?
Here are my suggestions:
1) Put a 50% Discount over X-Items in Obsidia Department Store pre-Corey.
2) Make Fern not a complete death fodder against Aster & Eclipse #1 by giving Lombre decent Water-type moves.
3) Don't let Taka lead with Klefki. It may sound like a good idea with human battlers, but alas, our AI isn't nearly as competent as actual humans.
4) Consider either making Level Limiter mandatory for Insanity Mode, or aligning the limiter caps to the gym caps for Insanity Mode only.
5) Consider removing the level boosts for Clown Indra only.