In the US, there are legal questions
In the US, most ISPs (I use Comcast, which is among them) specifically prohibit sharing of your bandwidth with strangers in their user contracts. Thus they can terminate your service at any time if they determine that you are reselling your bandwidth or just letting others use it for free (businesses hosting open wifi access points have to buy their service under a different, more expensive business contract which allows this).
If someone is detected downloading child porn or even just downloading illegal movies from your open access point, you will have a very hard time proving that it is not you. Prosecutors, for example, will say that you opened up a public access site just to hide your illegal activities. At the very least, your home will be searched and your computing equipment seized. People who host Tor exit nodes (an anonymizing protocol which shares your bandwidth with strangers) have had all kinds of similar problems with ISPs and legal authorities.
People who host open access points and Tor exit nodes usually have the best of intentions. Unfortunately, our legal system does not assume that level of good intentions.
A few years ago I used to host a fon open access point. Aside from the legal issues, the router hardware was a bit behind other routers (e.g., no gigabit ports), and the software didn't have many of the features of comparable routers. The situation may be better now, but fon just isn't as well financed as Cisco or D-Link and can't update its hardware and software as often.
