All inclusives are going to be different at different resorts. We did the AI at the Wyndham so that is the only one I can speak for personally. But usually all inclusive means unlimited food AND alcohol, but always read the details of your plan and resort. Some resorts like the divi are only all inclusive (you can only do AI there, no other option). The Wyndham is a resort that offers an AI package so not everyone who stays there does all inclusive.
For the Wyndham....we had unlimited food and alcohol (premium brands...anything more you'd have to pay for). Some menu items at the Wyndham had a $3 or $6 surcharge, but these were clearly marked and was because they were dishes that you wouldn't expect to find on an all inclusive.
We definately had more food than we could eat at the Wyndham....no way you could go hungry
The important thing is just do you research and to know the details of your resort/package. For instance....if you drink....know what brands of alcohol the resort does/doesn't offer.