In essence, the absolute quality of AI as an opponent really depends on the type of game, the amount of time spent on developing the in-game AI and the CPU power/allotted cycles behind the AI.

On a simple board game, like Chess, the home computer powered AI would always win because it can crunch the moves better than a human ever can and the algorithms to do so are very well known. 

Respectively, on arcade shooters (most FPS games), an AI can have perfect aim (assuming things like projectile speed >>> player speed) and perfect "vision" in which case you can also end up with a situation where AI will just wipe the player out of the game, assuming the in-game resources are even between AI & the player. On the other hand, for example, plug vision recognition into an FPS AI and you'll soon run into real processing problems and the player suddenly is the favourite (for now!). 


On RTS, AI running on a normal PC can be superior or inferior regardless of whether it follows the rules or not. The complexities of the game decide whether balancing the AI requires handicapping or whether it has to "cheat" to provide a decent challenge. Same goes for 'complex board games' (turn based strategy games such as Civilizations, but also Go [for a while on home PC]) and certain other genres, such as flight & racing simulators, where the superiorities of AI – near instant reaction times, superior number crunching, perfect memory and full knowledge of surrounding environment – can be overshadowed by the complexities of the game mechanics.

The fact of the matter is that *Standard-Game-AI* running on a home computer with similar to human limitations (e.g. AI sees/hears what you would see/hear) will almost always lose to player beyond simple board games (for now!). On the other hand, make the rules simpler, add extra in-game knowledge (e.g. AI knows the environment, including where you are, all the time), crunching power and AI development time and the human might end up losing the game. 

Overall, the question of ethics is in my opinion quite irrelevant and misleading. The point of playing a computer game vs AI is not to win or lose, but to enjoy the experience.