I've used the 3", both types of 4", and the 11" flexwatt. I have yet to have an issue. I bought the tool for crimping the connections early on though so all my connections were done with the right tool and connectors that were designed to be used.
Now though I have moved on to professional racks. The vision racks use a heat cord that I don't really like and am now in the process of moving to ARS racks with their heat panels, which use flex Watt. I like how ARS puts the flex watt on sturdy steal panels that allow me to put a thermostat on securely. I also like the 9006 holds the largest of bloods and the 8514 will hold most other adult bloods and stacks perfectly with the 9006 and offers the nice windows on the front. Brian at ARS has always been great to us even when we ordered in the middle of a move.