I see it more with "professionals." Everyone I've known teaching at church or working with Scouts has been doing their best.
The *big problem* with RE, in my opinion, is that parents are half-hearted about their faith. I suppose getting the children to a class, most of the time, is *something*, but it's not much.
But dedicated volunteers who know their stuff DO try their best and are usually pretty good. Our BSA troop and our Venture crew are full of good volunteers.