As always, the issue is not straightforward. Wenger hasn't underperformed in terms of overall spend, relative to the richer teams. But he has arguably underperformed given the player resources at his disposal. In other words, there is good reason to believe that a better tactician, a more reactive manager who sets up his teams according to the opposition, and who doesn't seem to manage solely according to the training ground and the lap top would have done better with the players AW has. Of course, its Wenger himself who is responsible for bringing the technical talent that we have to the club, so the argument can easily become a little circular.
The main focus for criticism, IMO, is that the manager has squandered his financial resources by indulging experimental players. Without doubt, we pay untested and mediocre players too much - and they are millstones around the club's neck. There is good reason to suppose that had the manager behaved more judiciously, and had a more flexible wage structure (this wage parity has done nothing to foster togetherness and loyaly), funds would have been released to allow the club to keep its best performing players.
Anyways, for me the buck stops with the manager. The board and owners are not without blame, but the criticisms above are down to Wenger.