I have not read too many success stories of those with electric brakes on boat trailers. I think
@Speedling is the only one I know of. I have read several stories of those that have tried them or past experience, they are great on dry land for sure. My anchor welder is a big fisherman with a center console, and he has oil bath hubs and tells me many of their group tried electric brakes a few years back and ended up switching back to surge brakes. A number of manufacturers have tried electric brakes on their boat trailers and most have switched back to surge units, but I am sure there are upper end trailer/boats that use them, because if money isn't a consideration there are ways around the water/electrical issues by using upper end equipment.
@Speedling , post up what you have, where you got them, and your actual success, we need a base line on this if guys are going to consider them. And...there is more than one type of electric brake setup out there, so results and reputation is speculative if you don't know what they are using.
The hill thing is what it is. If you feel/hear the boat riding against the tongue, then you know your brakes are dragging. It isn't going to drag on the average downhill section, it is only going to ride against your tongue on a steep grade. Average hills won't cause this, but you can still get heating on shallow hill grades if...your on a downhill grade and you actually apply brakes, then that will force the weight of the boat and trailer into the tongue and it won't back off until you accelerate. So in hilly terrain, technique is important to keep the boat off the tongue on downhill sections. The account I posted about out around the Sierras that had issues actually blocked his actuator on several trips and just went slow and took breaks on another trip. I worked around it just by paying attention and knowing if the trailer was actuating the brakes, if it did, you would pull over and stop, and then start up against to extend the actuator and release the brakes. Very steep grades, he blocked them. I had a caliper freeze up once on a vacation trip to Arkansas. I pressed the caliper out of the way, bled some fluid out of the lines to force air into them, and disabled them to prevent another lockup. I did the return with no trailer brakes without issue. That would not be the recommended method of routinely dealing with hilly terrain, but if you happen to have a long downhill grade in the mountains, go slow, and consider all options. Having the brakes drag for several miles isn't an option, you will burn your brakes up. The best thing you can do is to pay attention to your surge unit and know when brakes are applied. In and around lakes with pretty undulating terrain, it can be pretty common to be going downhill for a long way. And if you start and stop a bunch at corners or stops, then you apply the trailer brakes often, if the terrain remains downhill, and you don't get the weight off the tongue, they are dragging. So just being aware, gives you the ability to jerk the trailer a tad to release the brakes at every start.