Leaving aside the personalities and/or relationship dynamics involved, the main disadvantage of asking family and friends for feedback is that for many of us their opinion is worth diddly-squat. Their thoughts as an ordinary reader might be of some help -- provided they are completely honest, which isn't always a given -- in that being told they lost interest at one point, or they hated a character, may point you towards larger problems. But that requires them to be able to pinpoint what they did or didn't like and express it in a way that can suggest a path forward. But even if they are avid readers of the genre in which you write, they're unlikely to be able to help with technical issues such as POV, which might need to be sorted out.
A lot depends on what you want to do with your writing. Imagine that instead of writing you're talking of painting. If you're happy to bumble about and just enjoy yourself and perhaps give enjoyment to a few others at an annual amateur show in the local village hall, then all you need is family and friends to say if they like your paintings, and there's no need to worry about perspective or anything technical. If on the other hand you want to have something in the Royal Academy summer exhibition, you have to master technicalities so you need to learn and be critiqued by people who know what they're talking about. (OK, the analogy might fall down if we're speaking of modern art and unmade beds and the like -- think real art in this context!
)
If you want your writing to be the best it can be, my advice is to find people with whom you can connect, who write in your genre, who read a lot, and whose ability is about the same as your own, or preferably a little higher. One way to find them is to join a writing group in real life, if one is available; another -- and to my mind the better way -- is to cultivate members here and form a writing group with them, online if need be. The path to this latter way is to join in Critiques by offering feedback on other members' work and then by putting pieces of your own work up when the magic 30 posts have been achieved. From there take it slowly, make friends, and find members whose opinions you respect and whose work you'd be happy to critique in your turn -- because all of this is dependent on reciprocity. I'm pretty sure that this is how Jo has found many, if not all, of her beta readers. (EDIT: Ah, Jo has replied while I've been typing, so that's sorted out!)
For what it's worth, my writing group experience is mixed. The small group -- there are only 3 of us -- which arose from our membership here is excellent for feedback (and for friendship, general gossip and cake!!) but erratic in actual meeting, so we may go months without getting any critiques, though we then can plough through up to 10-20,000 words for each of us, and sometimes more. In addition, we can give help and advice online if it's needed between meetings, and since the other two have MAs in creative writing they know what they're doing. The local group I joined at the end of last year is religious in having meetings every month save in the summer, but pieces are limited to 500 words per session, the other members are a very mixed bag of abilities, and most of them are writing YA or children's books which I don't read or know much about, limiting my enjoyment of the work involved and my ability to help beyond obvious issues.
