Without regards to the cruelty or behavior problems, i will say the they flat out don't work.
We have three neighbors on our block that have installed these systems. One has a black lab, one has a Yorkie/Schnauzer mix and another little fluffy thing (not sure what the mix is) and our direct next door neighbor has an Australian Sheppard and a Bichon.
The lab no longer wears his shock collar because it really didn't keep him in the yard in the first place. The Bichon ignores it, but the Australian (very close to Sheltie temperament) has become nervous and prone to "accidents" in the house. This is all since the collar went on. The other two up the block are constantly chasing rabbits or ground hogs across the wires and getting zapped.
Now, we live on a low traffic cul de sac so there's not a huge amount of danger of getting run over, but there is a large animal population from chipmunks to deer and many dogs, some of which are not entirely friendly. Any one of which is just too interesting to a Sheltie not to investigate.
You could argue that these people didn't use the device properly, but we've had Shelties and Sheltie mixes for over 20 years and with less effort than these people have put into the shock training, i have taught my guys where in the yard they are permitted to go and where they may not go. FOR THE MOST PART, they behave, but it's always that one time that could get them hurt, which is why we have a fence.
Now let's approach the question from another direction. Why do you want to go with an invisible fence instead of a real (physical) fence? Is it convenience? Expense? Aesthetics? I would hope safety were in there somewhere, which an invisible fence can't provide 100%.
Real fences don't have to be eyesores. We have this style of fence ...
http://www.softpuppywarmhouse.com/wp-content/uploads/2012/11/22.jpeg
Against the landscaping it is very unobtrusive.
To sum up, you can do as much, if not more, with positive training with the same amount of effort as the shock collar with a Sheltie, but to be as safe as possible, a real fence is the way to go.
BTW, the three neighbors who have these fences ... and are constantly chasing their dogs ... still claim they work. I think they are embarrassed or ashamed to admit they wasted their money.