I think there are several missions that suffer from spike of difficulty(at least on Insane).I will start with those that i don't think it's really an issue for them to be harder than others:
Her Finest Hour - It's difficulty comes from extreme complexity of mission. Player has to learn a lot, and possible replay few times, to chose best course of action. Last update made it even more difficult, with 2 extra corvettes, Fighter Sweep, and ability for enemy pilots to detect you if you get to close. Player has lot's of tools, and overall missions is very fun, and getting killed feels like you really made some mistake, instead of feeling cheap. I think i won this one over 50 times (not even kidding), and overall for me it's the most fun mission i ever played in any freespace mod. Becouse of that and becouse of grandeour of mission i feel difficulty spike is appropriate.
Delenta Est - Similiar case to above. Biggest problem is that mission is really long, and getting killed near the end really sucks. But ability to load game really helps a lot.
Darkest Hour - My only problem is with last part, when Diomedes arives. To be fair I guess i got used to, that in other version(before diomedes update) Indus could handle it alone. On insane on the other hand, if you at this point of the mission lost most of your wingmans, or need repair, it puts you in really bad spot. Since you need to kill at least one beam to get Indus to win, and you don't have access to grimmers/archers, you have to get really close to it which can be very problemtic
. I guess i'm very salty about this one just becouse I lost multiple times there just becouse i couldn't pull away fast enough/unlucky beam slashed me from far, and saves were kind of broken at first (upon loading the Base would just explode). This problem was fixed when i restarted game though.
One Future - This one feels really like it's impossible one insane(at least if you try to kill this capship). Even if i manage to kill it before it kills me, shortly after wing of fighter/bomber kills me of almost instantly. But than i guess i didn't get used to custos, and didn't try that much.
The Plunder - This is only mission that feels really unfair. If I get even close those cruisers and CV they just shred me with their turrets, since rest of friendly fighters just charge strike into them and enemy stick close to cruisers at start, you can't really pull enemy craft away from them. Also gamma is doing terrible job at disarming Siren so it's your priority to do it for them. I finaly found a way to easily beat this mission just by charging at Siren, disarming him, and than keep away from fight for the rest of mission(Aside from disabling logistic ship ofcource). Also since you don't have access to uriel yet, you really need to take paveways, instead of Slammers and Grimmers (not even sure if grimmers are aviable in this mission)-> so you can't really play as sniper here
Why i think difficulty is a problem here? I don't think you can really win here on Insane by actively participate in battle. In most missions you are discouraged from just jumping happy into formation of enemy ships, but here exactly this happens. I have to watch, how my wingmates are having fun, while I have to stay really far away to not almost instantly gimped by cruisers, which feels very anticlimatic.
For some reason, this mission seemed way easier before this update. (I guess too easy, since i remember you didn't have to do almost anything to win).
I really hope that my post doesn't look like rant