First of all,
2x/3x Health in Damage
Second, on the opposite end of the spectrum, there are "do 2x/3x your health in damage" missions for which SPGs qualify. WTF are these even a thing? They're a complete joke. I completed all of them (U-2, A-10, C-13) with honors on first attempt, by, wait for it, using SPGs. There are some missions (e.g. U-8 & A-1) that require you to earn "Fire for Effect" medal, which disqualifies SPGs. All of these missions should require that as the primary condition.
Next, there are inconsistencies in spotting/assistance mission difficulty. Some are very hard, and basically required you to platoon with at least one other good player (U-1, B-9, A-3), but some are a complete joke at less than half the difficulty of the previously mentioned missions, for example: U-12 & C-12 (the latter I completed during a series of absolutely abysmal games). I really cannot get over the absolute lack of any challenge at all for these latter two missions as compared to the former (7 vs 20 assisted kills in 10 games).
For the spotting/assistance missions that require an average over 10 games, they should really allow your own kills/damage to count and increase the requirements a little bit, or split the requirements into assistance and own damage, while reducing the assistance component. Many, many times I hold off securing a kill myself, to the detriment of my team, just so I can try to meet the mission requirements. Light tanks should not be 100% passive. They are most effective in cleanup and eliminating weak, isolated enemies. You shouldn't be punished for doing the right thing to help your team win.
These missions should reasonably be attainable solo with a similar level of difficulty to the other kinds of missions, which just is not the case right now. I think B-4 is an example of how they should be: Challenging conditions, but a small number of games.
Some factions have missions that require 30 internal modules and/or crew damaged in 10 games (A-8, C-4). These are incredibly frustrating and very luck-dependent. You might think SPGs are the best way to do them, but averaging 3 per game when only internal modules count is actually quite hard, especially for Coalition which has fairly wimpy SPG guns. When I say internal that obviously means not tracks (which would make it trivial), but also not guns(!!) and I believe observation devices don't count either.
For Alliance these shouldn't be too bad if you have a 183mm TD, although it will require a lucky streak of games with some big numbers. You also have potential of T-49 or Sheridan with derp gun, and E3 or E4 with HE spam, not to mention the larger SPG guns.
For Coalition it was a nightmare. I tried with BC 58, Progetto 65 with Deadeye (for non-HE) and a large HE loadout, and EBR 105. I was finally able to do it after a week of attempts in the Standard B with mostly HE, on 3 lucky games in a row with massive module damage numbers. The only reliable way to do it IMO is Foch 155 with HE, but that isn't attainable any more (aside from Black Market).
A much better example of how to create these missions is U-7, which only requires 3 internal modules/crew 3 games in a row.
This is another type of mission that is frustratingly specific to certain tanks, conveniently premium tanks. Prior to the Obj 279(e) series the Coalition series of these could be completed with the AMX 13 57, which is an absolute god. You need tier 8 for Obj 279(e) though (C-6), so that's out. Fortunately the EBR 75 exists, which made this mission a breeze. Before I remembered I had that tank, I tried many times to complete it in the Progetto 46 & T 50 and fall short.
For some reason, Union gets a much harder version of this mission (U-11) that not only requires a much higher number of pens (80/120 vs. 75/90), but also only counts games where you had at least 5 pens. I haven't done it yet, but I think that mission will be virtually impossible in any tank besides the LT-432.
Bloc gets a really weird version of this mission (B-8) that requires 7 penetrations for 3 games in a row, which isn't awful, but the bonus condition is 12 penetrations. Are they completely insane? That would be a challenge even with the E25, but this mission is tier 8+. Similarly A-14 requires 4.3 vehicles penetrated per game over 7 games, which is tough, but the bonus condition is 5.7 per game, which is pretty outrageous to maintain for 7 games.
Overall, the Obj 279(e) missions seem like a good challenge (at least the faction I'm primarily working on, which is Coalition). There are some serious WTF? aspect tho, as mentioned above. Some missions are disgustingly easy and serve no real purpose. Far too many missions rely entirely on luck, and some actively punish you for doing the right thing (blocking missions, and assistance kills).