FUE: what are the risks of using my beard for the donor area?

Beard hair is not the same as scalp hair.  In general, you would want to have matching hairs.  Beard hair is not always the primary option for donor hair.FUE will always leave some form of scarring.

Beard hair is a great donor source, assuming you can grow an adequate beard underneath the chin. The majority of beard hair usually consists of single hair follicular units so a smaller FUE punch can be used for this procedure. In general, smaller punches tend to heal with a smaller scar. In my experience, the FUE extraction sites are nearly healed by 4-5 days following a beard FUE procedure and most patients can resume shaving at that point. If you have any concerns, I recommend further discussing this procedure with your surgeon. 

If you use you beard as donor for the than one might get some hypo or hyperpigmented small dots depending upon the size of the pinch used...we recimen to use 0 .7mm punch which gives very nice healing and very fine scar after even hard to judge..
One thing more mostly beard contain single FU but their diameter is better than scalp hairs..

Beard hair and scalp hair are different. Body hair for hair transplant has a much lower survival rate because of the technical difficulty and trauma to body hair. In addition, FUE scarring will be present.You are correct, you should worry about the appearance of your neck. Do your research well and avoid a bad hair transplant outcome.

When you reach the limit of your donor area hair, then alternative sites such as the beard below the chin is a reasonable option but this hair is not scalp hair so it must be burried in a part of the transplanted that does not allow you to view the actual hair caliber.