Horror stories occur for one of two reasons:
- People get banned for doing more than multiboxing and lose access to all of their accounts. Sometimes bans happen but if you weren't doing anything wrong the appeal process tends to sort this sort of thing out if you were utilizing caution in your play. There are certain things GMs check for i.e. conversation (whether you're chatting in game, which I think is a stupid check personally but one of the many areas they look to determine if you're botting), they teleport you around and you just run right back to your group (which is something a bot would do, according to Blizzard), etc.
- People spend all of this money (gold or $$$) to set up a 3, 4, 5, 8, 10 box of characters which adds up fast in terms of startup costs and then don't want to put in the work to sustain the accounts. I myself have had to take breaks because I started to find group farming and gathering boring and wasn't making enough to support per month (but had several million in the bank). These people burn out and lose out.
All that said, the two key takeaways are don't be stupid by doing things you shouldn't and make sure you actually want to do this. It's okay to take time off but remember when you're subscribed that you'll likely need to spend probably 6-10 hours per month gold farming hard to support the accounts.
Last thing, as far as programs to use, nothing compares to ISBoxer. It's more than worth it but does come at a real money expense. If you're thinking about doing this, factor in the cost of ISBoxer to your decision because using it (a simpler, more effective program) will increase the likelihood of you sticking with it.