The problem is:
1) There is the possibility of abusing cheap units like troopers to break the balance on the campaign map (like a glitch). Like Stabbey wrote: you lose less units weithed in gold than the enemy with the bigger army....
I agree. However, if you adjust the autoresolve in such a way that it prohibits such a possibility, this problem you mention will remain NEVERTHELESS. Then, your opponent can still attack you with one single trooper and choose to fight the battle - the outcome will be the same as it is now with autoresolve. Just adjusting autoresolve does NOT eliminate the problem of ninja troopers.
It does. If your lonely ninja trooper has a 0% chance to win it's just pointless to send him at war....

2) There is the intention to "force" people to engage in RTS which is bad decision in itself. If I don't like RTS that much there shouldn't be any mechanism "by design" which let me worse off. At least the game should give me the possiblity to avoid that. Else people who don't like RTS gaming that much will be forced to play the battles on the field which is perhaps not what they initially wanted. Sure, you can always reduce game difficulty. But that's not the best solution imo.
Why do you think that you are forced to engage in RTS by the autoresolve now? For an
average player it yields pretty much the same results as actually fighting the battle. If you are better than average, you should either do battles yourself to achieve better results or just accept autoresolve. As I have already written, that is exactly the point of the player's ability. In contrast, if you adjust the autoresolve in such a way that it yields
better results than an average player would achieve in the battle, then you will rather
force the people to do autoresolve rather than fighting an RTS.
The problem is the that when you attack a country with a huge army which has an 80% to win you still lose quite often. If not the battle so at least MANY units and as Stabbey said: the bigger army loses more troops weighted in gold than the small army. THAT'S JUST A BAD DESIGN DECISION. It breaks the balance on the campaign map. And yes, the problem is the RTS simulation in the background when autoresolving. It takes away weigth from my units on the campaign map because there can be new units build in RTS. Bad that's the wrong transition from campaign map to RTS.
When you atack with a big army, you loses should be smaller or equal the enemy losses at the maximum and the other way round. If you attack with a small army you should lose when autoresolving. That's why there is an RTS mode after all. To give you the chance to win battles which a bad percentage to win or a 50/50 percentage to win. At least that's my point of view. Well, if you bad at RTS, you just shouldn't attack with a small and insufficient army...

3) The mechanic which motivate yourself to engage in combat would be even better WITHOUT the RTS in autoresolve. When you have a clear 0% chance to win (or 5% or something) and you still want to attack a country there is no other way than engaging yourself in battle. On the other hand: with RTS involved in autoresolve the motivation to engage in combat is ALWAYS the same. Is that better than the way without RTS involved in autoresolve where you are more motivated to engage with a small chance to win and less motivated with a big chance to win?
Why do you assume that you get a better result when you fight a battle with a big chance to win rather than autoresolve it? As far a my experience goes, I always get a comparable or even a worse result when I play RTS rather than autoresolve.
The problem is the transition from campaign map to RTS and back. Even if you win a battle in RTS your loses are often way too high. I think that's even the biggest problem of the complete game so far. Total War avoided that because you can produce troops in battle. So the transition from the RTS map to the real time battle an back is quite easy. It's simple "count the death units" maths there. But it's way more complicated if you can build units in RTS battle.
I think that the RTS battle and the campaign map battles should be somehow "decoupled", so exactly the opposite of what Larian has done in the last updates. It's just bad balancing so far, just look at STabbeys examples....