Some might argue that buying US forces does give you an advantage.
Then that is a balance issue, not a design issue.
Balance issue: USF is overpowered
Design issue: buying USF gives grenadiers +10% accuracy OR USF is designed to be overpowered on purpose.
No, but numerous times in the past DLC commanders have. Hell, DLC commanders still give major advantages. Just look at how often Fortifications was used as OKW in the most recent 1v1 tournament. Not to mention the abominations of old - Tiger Ace/Windustry.
If only you could trade in duplicate war spoils for new commanders... might finally fix the stupid DLC issues we've all complained about since day 0.
While I do not agree at all with Relic's DLC system or their warspoils system, this is an unfair comparison to make compared to Dota/Lol. Neither of those games have anything locked behind a paywall. Every champion is freely obtain(ed)(able) in Dota2, only cosmetics are purchased.
In League of Legends, you can choose to spend real money on champions. However, the pricing on the current champions is completely reasonable and something that a competitive player can easily attain simply through playing the game.
http://www.lolking.net/champions/
According to that chart (at the time of posting), in the top 10 champions;
4 champions priced at 4800
2 priced at 3150
2 priced at 6300
1 priced at 1350
1 priced at 450 (!!!) --> ~ 80-120 IP per game depending on time, win/lose, etc.
Stat boosts in the form of runes are IP only and CANNOT be purchased with real money.
Also, unless DLC commanders are designed to be overpowered, it is more of a balance issue than a design issue. Just nerf USF if it is OP. Failing to nerf USF simply because it comes in an expansion despite its oh-penis is a design issue.