For me, these are the worst immersion killers:
1> wrong use or no use of skins
2> wrong loadouts
3> untested missions
4> fantasy tactics & targets
5> lack of research
as for 1:
IL-2 is an eastern front sim and most default skins for European theatre axis planes have yellow noses, wingtips and fuselage bands. That color was not used outside the eastern front with the exception of some yellow noses in the BoB.
When campaigns turn out to have default skins for everything except the player's flight/ squadron it kills the immersion because it has obviously not been researched properly. Once you know about tactical markings it becomes impossible to overlook.
as for 2;
Long ago I decided to make a Bf-110 campaign for 2 reasons; one being the thought that for every other type there were already plenty of campaigns but not for that one. The other reason was the stock campaign for Bf-110 in version 2.04 and later ones. Besides the fact that it was rather boring, it also had you attack tanks with anti- personnel bombs and other impossible loadouts that seemed to have been put in without thinking. Also, the transit altitude was always the same, just like the running into an enemy patrol after 80% distance to target in EVERY mission. Besides that, the enemy AAA was always exactly in your path at the front line, damaging your plane in 50% of all sorties...
Worst of all, it was 63 missions with 0% variation. Ground pounding only.
as for 3;
I tried a campaign someone made around a certain theme and in the very first mission one had to intercept a He-111 which was doing recon according to the briefing.
All you had to do was follow the leader. Well, the leader flew off at top speed and disappeared in the clouds because the He-111 was in the clouds.
The weather type being rain, it was impossible to see the leader and
Had it been tested, the mission builder would have found out soon enough that this was a mission impossible. (Same mission had eastern front skins on all aicraft) . Also, consider this; photo recon while flying in 10/10 cloud, in the rain? Come on.
I never continued the campaign.
as for 4;
suppose you fly an IL-2 mission and the whole squadron of IL-2's fly at 3000m.
To me, that kills the immersion because the IL-2 was never used like that. It takes less than 10 minutes to find out that the VVS used them at altitudes below 1000m to avoid interception.
Then, arriving over target (let's say, a Luftwaffe airfield) in your IL-2, to find out that the targets are lined up perfectly and planes are parked wingtip to wingtip, that kills immersion, too. Targets should be dispersed and/ or camouflaged on airfields. However, if it's a VVS 1941 airfield, they should
be lined up perfectly. See also under 5.
as to 5;
like some exampes I already gave, lack of research can be an immersion killer. For instance, if you fly a FW 190 over Stalingrad, that is incorrect altogether. There were no FW's at Stalingrad.
It's obviously no problem for those who no nothing about that kind of a thing. But most of us do, and because the information is all available either here at M4T or elsewhere, the killing thing is finding out that the mission builder did't go through the trouble of finding out.
Another element of research would be that a good mission or campaign has correct dates and front lines. If you fly an eastern front mission which has blue in the east and red in the west, I'm done before I even get started.