... surely a product would not be still around if it was 100% grade A junk, right?
Oh I wish this were true, Samual! Junk is knowingly left in the marketplace and sometimes even injected into it by businesses who are trying to make a quick buck on the inexperienced. Think "As seen on TV"... sometimes they work great and sometimes they fail after a short time. Ya never know!
I think the same could be the case here. I bet there are people still using cable heaters to this day, and others who abandoned them long ago.
Here's my theory, and I do not have a lab to experiment and prove my theory, so it kinda is what it is... a theory. It is not useful as a solution for your question about heating the substrate in an emersed set-up, but I am assuming that you don't intend to leave it emersed and you will fill the tank eventually.
I believe that water exchange is important just as oxygen exchange is important in the substrate. It moves substances around the tank and into the substrate where it can be processed by bacteria, invertebrates, and plants (at the root level). I can actually see it happening with my eyes to a degree, and that is evidence of the exchange as well. Have you ever watched the water near a submersed heater? As it's warmed, it moves up the water column. It looks kinda blurry and wavy. Those water molecules above the heater are being displaced by the warmer ones due to the fact that heat rises... thus they are always moving... even if you can't see them moving. Filters and power heads also move water around the tank as do swimming fish. Particulates (beneficial and non-beneficial) are being moved around the tank all the time. Gravity and heat differences in the top layer of water vs. the bottom layer move both water and particulates around constantly.
The substrate, if it is aerated, will have pockets of space that will "catch" these particulates, water, and gasses. That's one argument for not using sand without MTS and/or other aerating inverts. It can become a breeding ground for anaerobic activity without aeration due to compaction (read: low or no flow). Larger substrate particles like gravel are less likely to compact because of their shape and therefore can be deeper without consequence. Still, there is a depth at which the flow is so slow or even still that it becomes anaerobic even with gravel.
UGFs were designed to eliminate that danger by keeping the flow steady. There are still dead areas where the UGF gets clogged with debris, algae, or even plant roots. It's not that they are bad... they just aren't worth the trouble over time for the same reason Homer said... You have to break the whole tank down to clean them. They are still out there, and they are still used, but they have lost popularity due to advances in technology.
So here's what I'm saying in a nut shell... as long as there is a heater of some kind, a filter or a power head, there will be enough movement to create a beneficial exchange of water, oxygen, and particulate matter within the substrate. By placing the heater inside the substrate, you could actually move these things up in the water column removing them from the substrate instead of moving them down into it simply because heat rises and the source of heat is at the very bottom of the tank. Again... theory.
What I think you need more than heating cables are MTS (or other inverts) that can aerate the substrate for your plants and a warm room for your tank during the emersed stage, and a regular aquarium heater when you fill it up.
MTS would be much easier and cheaper for sure! Hey, I like this winning thing hehehe
