Back when it was new, I started playing Winter Assault, but it got on my nerve very quickly.
Now that it's on GOG I gave it a second try and it pisses me off again.

I love the original DOW, but Winter Assault ... I just can't stand it.
Convois here, bunkers there and masses and masses of Orks, it's just no fun.
And the imperials are unlikeable idiots (if I hear "Where is his will needed" one more time ...)
The game can't handle the large troop sizes well either.

Currently I am on a race with the Eldars to the Titan, using a convoi.

Does it get any better at some point? Is it worth suffering through the next levels or should I skip the whole expansion?

Next levels? Campaign have 10 missions (5 for Order and Disorder) and 2 missions (1 per Order/Disorder) where you chose side (4th missions of both campaign).
Whole idea it is large hard missions. Half of original DoW campaign was "Press X to win" (select and send troops to other side of the map).

Its not about difficulty at all. It is not "if this game is too difficlt for you". It is "if this game not for you". The whole genre not for you. Maybe PC gaming not for you. RTS supposed to be not the "press X to win" games. You deal with map/resources/time (capturing map points), defending against attacking enemy and at same time build own base.

Winter Assault been AddOn for DoW just add few hardener mission to first 11 ones. Short campaigns. If you chose only one aide of four it will be just 5 missions for you. Eldars most easiest one since 4th mission for them going easier. This one mission os fruu only for Impire cause you get access to own base portioned based on how far you did get.
AddOn indeed much hardener and longer for each mission cause there is less missions than in original. If you chose one side it be less missions.

Last 5th mission (you are on 4th) will be same hardcore. You will be in the middle of the map defending against foes around. With new enemy coe at the end and this be even more hardcore.
That the whole idea of this AddOn.

I mean. This is about basically DLC content. Who will buy it if it be just same or less content than original game? Back in '04 teenagers kids whiene about games be too easier. That campaign is just tutorial for multiplayer game. Devs give us almost-same-but-newdiffer campaign.

Time changed. And later they release casual A-to-B corridor Press X to win Dawn of War 2 (and then 3).
If you did not agree with me about how DLCs should be, maybe DoW 1 is too much old for you. It can be vise versa opposite more easier variation of main campaigns. But not for DoW1 where original campaign already be too easy.

So in short: It won't get better.
"too much old" ... I played the orignal version on CD.

I played some RTS back in these days, loved DOW1, but Winter Assault was - simply put - something that caused headaches. And no, it's not about the difficulty, you said that right. it's about the playstyle. I expected a continuity in the spirit of the original game which it does not have. It's just unlikeable and the AI that can't handle large amount of troop movements is very annoying.
I am more a special unit surgical guy that one that uses the big hammer. I prefer strategy and RTS with smaller army sizes, that's what I loved so much about the first DOW. It was not a mass slaughter like the C&Cs and similar games of that time. Sadly Winter Assault is completely different than the base game was in that matter. I was just hoping that at some point it would change and match the tone of the original.
I don't care for multiplayer in tactical and strategy games at all - never did (one exception: Back in the days I did enjoy SW Supremacy against a friend once in a while). The campaign is the only thing important to me. If they can't get that right ...

Thanks for the information.

Well, my IMO it opposite most disadvantage of the game. Trying to be like Wh40k canonical game...

As for AI. Agred with that one. For pathfinding feature that... hmm... units moving in large groups try stand far away from each other. Maximum population army by one move command will spread across half of the map. And so for AI in Dark Crusade and SS.
For AI opponent it was better for DoW+WA.

No else disadvantages i guess. And if you ok with small armies battles tabtetop-like - DoW should be good for you. Including WA.

Another disadvantages is enemy are not limited at same level It have at least two differ bases with separate unit limits. But again that going worse in DC and SS on hardest difficulty. Its just skirmish maps when you are one army against two.

As result it is not lore-canonical for Wh40k. And not even tabletop canonical. Whaever faction you chose you deal with more populated enemy. Eldars overwhelm Imperial Guard, heh.

But that my opinion a bit differ from your one. I think this small armies - which one indeed main idea of original DoW - is disadvantages and DoW2 bring it worse. For people who like original idea DoW2 are better. Anyway campaign there more casual.
So for me WA looks like some mods that increase armies. Still limited for player. This one 4th mission if you play as Impire you get allies bases so increased army. But going worse to the end of the mission.

Best solution was in Warcraft 3. There is one mission in Wc3 Expansion allow to use multiplayer mechanic to control allied base.
And Winter Assault bring same feature to Dawn of War. There even more in game engine.
So debug mode or cheat-trainers allow to do that.

Dawn of War 1 try to be that small\average armies tabletop game. But it wasnt. Not DoW, not WA DLC, not DC and SS. Sooo... Khm. DoW is a bad designed game...? Maybe we did play it for visual (blood blood BLOOD!!111)?

Dark Crusade and Soulstorm bring even more limits to more looks like tabletop Wh40k. Imperial Guard can have only guardsman squads mostly in SS. It was not for balance or make game be like small army tactics. They do that exactly to try keep it Wh40k. Well, DoW is too much skirmish\multiplayer game. All DoW1-2 singleplayer's try to be something 'in between' and all failed it. Too much large armies for be about small armies, too much small to be big fightin', with AI have doubled units anyway. DoW2 singleplayer are same. It was again too much small trying to show big battles and again AI opponent in campaign have separated doubled armies with separated unit limits.
Winter Assault are not big exception from it. They try show big conflict with small armies (for player).

I don't know much about the table top games - except StarQuest, which as far as I know has not much in common with the rest.

When it comes to WH40k, I am mostly into books. What I would really like in the WH universe would be a role playing game in the style of Ian Watsons books. Rogue Trader might be something interesting in that matter, even if it's setting is a little too far off for my taste, but it's better than nothing.

Horses for courses but WA is much better than DoW in terms of level design, normally seen as the franchise peak of single player - sadly the original Blood Raven plot does not even continue until Dow2 : Retribution. The above poster is unfortunately right that DoW campaign is spam -> attack move.

Dark Crusade and Soulstorm have narratively very poor campaigns, but certainly for soulstorm the mods are good to expand the game - Unification brings in a lot of the WH40K universe to skirmish, including an Ian Watson character you may recognise. You will want the steam release for that though, which is £3 and better than this one.