Are not inherently overpowered. They play perfectly into Nod's general style of subterfuge. Since Nod lacks the vehicular punch of GDI, they are a vital asset on many maps.
However some questions about balance are raised by a couple of telling observations. I'd be curious to see some actual numbers to back this up, but I've noticed a couple of interesting things:
1) On maps with no base defenses, many players switch teams to Nod immediately on start. I feel like I don't see this on maps with base defenses.
2) On maps with no base defenses it seems like almost every building Nod destroys is with an SBH nuke. At the very least, the first building or two often goes down that way, which tips the balance to Nod's favor. The overwhelming (and successful) usage of a single tactic suggests to me that this tactic, while perhaps not unbalanced, needs to be examined thoroughly. I certainly don't remember this being the case in the old Renegade.
As I said above, I don't think this means they are necessarily overpowered. But, in a few cases, some quirks of map design lead them to being more powerful than they should be. There are a couple of spots (particularly, the Weapons Factory and Power Plant) where a beacon can be placed that is very difficult to disarm. These are places that it is difficult to get to physically. The SBH has the luxury of taking their time to get to these perfect spots, while the defenders do not. Too often I have seen 4 or 5 engineers/hotwires stare at a beacon as it counts down, unable to get to it. To me, that's a problem. An SBH nuke should be successful because of skill, timing, and coordination. Not being able to stick it in a hard to reach place.
Additionally, and this applies to both teams, some of the new maps (Gold Rush) have base layouts which are quite large, which makes it much harder to find and disarm a beacon within the time. Perhaps a longer timer on these maps is in order?