You are correct, but the entire game can be summed up under “you can only win if the DM lets you”. At any time the DM can do anything, so this shouldn’t apply as a specific problem with this combat style. CAW is just as fair as CAS…the DM is the factor where fairness comes into play and truthfully if you have to worry about the fairness of your DM, then there are more serious issues at hand at your gaming table.
I think there's a difference. A lot of the things that players can do in CAW is, by its nature, not going to be covered by the rules. There's no "Bees are stunned by setting fire to the forest" rule. There's much more room for DM interpretation and having to get by his "do I think this crazy stuff work" filter.
For CAS, the rules can be much more concrete and both the DM and players tend to rely on those rules. There's a lot less room for DM interference or interpretation when the barbarian charge-pounce-shocktrooper for 100 damage. The feats and abilities the player are relying on are explicitly spelled out in books.
The DM can of course still throw out those books and say it doesn't work like that, but that's a much more explicit use of his DM power (and the players know it). It's the nuclear option compared to "I don't think the player's plan works and here's how the monsters will respond" which is a much more subtle use of the DM's veto.
The DM may not even trying to be deliberately unfair but the fact is that there is going to be a lot more DM judgement calls in a CAW playstyle. DMs are people and their judgements are going to be subjective, influenced by their background and knowledge. One DM may think a player's plan is not going to work while another DM may allow it to work.
Also, I may be incorrect here but does it seem easier to breed a DM vs Player mindset in CAW as compared to CAS.