I will actually do a bit of a breakdown for you (this is going to be a wall of text).
Tunisia
3rd Battalion Tobruk - Balanced. They are a full 9 person squad with a solid bolt action rifle , proper perks for what they do. They are worth their money. The only lacking feature is specialists, but these are one of the few that I’m okay with because the size matches the same as F2P.
8th Bersaglieri (FNAB Squad) - Underpowered, the FNAB is nice but with the lack of additional soldiers (i.e. Meat shields) they suffer from being rendered combat ineffective much sooner.
1st Grenadier (Motorcycle Squad) - indifferent, not as flexible as the F2P model, but with one more trooper, slightly more survivable. Still not worth it to use though (the only reason I did was because it was my only engineer until level 9 for Tunisia 
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1st Group Moroccan- Balanced , just like the other 9 man infantry squad for Axis.
WD Big 4 - see above for other motorcycle squad.
Normandy
557th Grenadier Regiment- At first, I wanted to say Balanced, had you asked me a few thousand battles ago, that would have been my stance. However now that I have been using both BAR squads for the US , I can now see that having the additional troopers and specialists makes all the difference in combat staying power. So, UP in terms of direct comparison to those squads.
941st Infantry Regiment (MP18) - UP, this squad might be good for your first 10 to 20 battles so long as you don’t run into anything too hectic, but they have zero reason to exist in late game environments.
9th Infantry Regiment - While useful for having the build speed perk already available to them, they are no longer viable now that I have a full Engineer II squad available that I can tailor make, it is obsolete.
60th Infantry Regiment - Same as the Axis BAR squad, ineffectual now that I have 2 7 person MG squads with the arguably better A2 variant.
Berlin
51st Grenadier Regiment - Balanced. The gun is good, the squad is a full 9 person squad. I tend to do just as well with them as I do with my mixed person squad. With the Caveat that I only bring these out once a RP is established.
1st Volkssturm Company - Woefully UP. I only use these when the battle is well at hand, or there is a specific spot that the Bolt actions can shine. Of all the 9 person squads available, this is the one that could use some customization. Adding in the same kind of specialists won’t bring them up to par with F2P, but it will get them closer.
Moscow
195th Infantry Regiment - a little bit UP, the guns themselves can hold their own, but against competitive SMG squads in this campaign, they are lacking.
452nd Infantry Regiment- Balanced. They can hold their own, and the sacrifice of not having specialists is made up for by being a full squad ( notice a theme).
206th Engineer Battalion - UP, the bolt action is unique to the Axis side, but beyond that there is no logical reason for it to exist. I can make a full engineer squad of 6 or a mixed with AT gunners that will have better guns, and loadouts than this.
Now onto the other points.
What would I change? At a minimum they need the same upgrade paths as F2P squads. The idea of premiums I have seen in other games is that they are equal, but different from their normal counterparts.
I only use the ones that feel as close to equal (with the exception of my Volksturm since they were the first premium squad I ever bought) with their F2P counterparts.
The only squad that is Pay to have an advantage (and this is observational from seeing them deployed in game) is the Moscow MG squad.
There is no need for An engineer II , nor for him to carry a non standard weapon. This, and this alone is a slippery slope.
(That and the PZ4 E with HEAT and Jumbo with multiple AP rounds).
Bonus
I kept vehicles out of this since they are unique, and different, but we need this Historical MGs on the StuG III, there is no logical reason to omit something that should be standard on it.
End of the day, this game is about numbers. We can call it whatever we want or dodge the reality, but having more soldiers for ‘extra lives’ or additional bodies on the cap point or train matters more than anything else. The longer a player can stay combat effective, the more he can contribute to his team.
I might get overwhelmed by better players, but if I end up having more it balances it out. It is why all campaigns I run have all of my 9 person squads in play, always. They are the wall that helps me turn the tide.
Hope this helps @Shiivex
almost all of them in Moscow, a bit less in other campaigns. I bought them as a collector and in order to support the game.