I'm went to Canada with IAEA and the experience was excellent. They only take people with a rural background, and do their best to match the trainee and host, so they both get what they are looking/suited for. Of course this can't always occur, but in general you will get someone who is keen, has some experience, and is willing to learn new things.
There will always be a small percentage of d***heads that get through, but that's the same with any group of people. If you have a clash, the organization can re-locate the trainee, however in my area the dozen or so trainees all stay where they were put without issue, and loved it.
I think the host families are always very good because a bad one gets pinpointed fairly quickly as they keep coming back year by year. The trainees are a year by year proposition, with no track record.
For both trainee and host family its "Life is like a box of chocolates - you never know what you're gonna get"
Don't expect a fully qualified 28 year old diesel mechanic, because you may get a 19 year old girl who spent six months in a flower farm. In my year we had that, and everything in between.