I used them lots many years ago…
Soramid (water) and Gaiamid (earth) were useless. Soramid damage is too low and Gaiamid requires something to sacrifice on top of being loaded which feels clunky plus Botanic and Sarcrosphinx are good support pieces that block the solo summon move (and I think the catapult damage might be too low for fighting mythics now?).
The other four were all brilliant. I found myself going for a controlling strategy using solo sleep from Banamid (storm) and then repeatedly attacking/healing with Neromid (shadow) and Vivimid (holy). Those three felt like the core strategy and using Sarcrosphinx to accelerate them made them a great wall against enemy aggression. Botanic and Gandalfrog also proved useful for extra loading. Lavamid (fire) was great with its piercing mortar but the poison revenge meant I had to pick between what type of strategy I was going for (sleep or piercing sweeping).
I found them particularly good in PvE, where it was easier to get the sleep control going and constant healing to keep them alive. Hellfire mortar double from Vivimid was my favourite damage move to use and even though it wouldn’t one-shot in PvE the damage was decent so it did the job.
In PvP they were hit and miss, as you’d expect. I did well with them back in the day, but Vivimid’s damage probably hasn’t aged well. The really noticeable thing is how you telegraph your moves to the opponent. Since you have to load a teammate it shows exactly which you intend to attack (etc.) with and your opponent can disassemble what you’re doing. You have lots of awkward experiences where you know if you load X then the opponent will just kill it and leave you in a bad position but if you load Y instead then they can just ignore X for a bit and your loaded Y isn’t doing enough that maybe you’re in an even worse position.
So the things to worry about in the overall design are the continual fast healing Neromid can offer (given they all have HG), Banamid’s sleep move offering too much control (maybe less risky now much more sleep counters exist) and Sarcrosphinx offering a relatively easy 25% accelerate. That said, I feel like the fast healing and HG is a key part of what they do that makes them unique and interesting. I hope to see Soramid and Gaiamid having more reason to get played in the setup besides loading teammates.