The other general rule is that you will always get better value by using points or miles for a perishable commodity with no alternate use.

i.e. the marginal cost to fill an otherwise empty room or seat with you is next to nothing, so the hotel / airline can give you something with a high nominal value without spending much

Exchanging points or miles for 'real' physical goods that cost a lot to manufacture and ship is usually a bad deal because the supplier faces a real cost and therefore cannot give you much in the way of value for a small amount of points.

Oddly, though, the buffet does fall into the category of perishable commodity because the marginal cost of you visiting it is next to nil.

If the hotel gave you a full meal it would cost them. That said, when we came back from Madinat Jumeirah (not a PC hotel!) in Dubai in January, we earned enough Jumeirah points for two slap-up three course meals with wine at Jumeirah hotels in London. One of these, in the Carlton Tower, would have been in the top 3 most expensive meals of my life had we paid the menu price. However, this is the exception to the rule!