It depends on what you mean by sensitive... ARs can and have gone thousands of rounds without cleaning. I've found them to be excellent and reliable weapons. Are there rifles that are more reliable? Certainly. Like everything else in life though, you make tradeoffs for that extra reliability.
One issue you probably will face though is at a $650 price point, you won't get a new AR that hasn't had some corners cut during its manufacture. Like any other design, when you start substituting cheaper parts instead of the parts originally intended to be used in the design, it can affect performance - although even most of the cheap ARs are still pretty reliable.
If reliability is your number one concern and you don't want to go any higher on price point, I would take a hard look at the Saiga rifles. If 7,000 rounds without cleaning is reliable enough for your needs, then you might consider buying a low-end AR and taking the small chance that it might need a little tweaking to run 100%.
