When I originally built my T-Bucket, I used the first version of the DeRale fan controller. That controller failed after two years and I replaced it with what I thought was the same controller, only to find the same issue you describe (redesigned to switch the ground side). Okay, no problem, installed the new version and move on. Controller failed again this week after be used in the car for 3 years. Both times, it appears the controller got hot and the relay had to be pried out of the controller's base. I'm running a single 2600 cfm fan which should be well within the capability of this unit. I've also installed an identical replacement fan thnking that was an issue, however unless both fans are bad it looks like the DeRale controller is 0-2 for me.
I'm thinking of trying the Painless unit (#30114 I believe) which switches the positive side of the circuit, so I'll be doing a little changing in the circuit again but it's not a big deal.