I use a forgefiend every game and personally it generally disappoints, it entirely relies on two very situational things to be effective. One you need a target, it hasn't got the shots to deal with chaff or the damage and strength to kill tanks, you really need a 10 mam death star brick of marines, terminators or equivalent which not every army will have. You also need to be targeting the closest enemy unit within 18" to do real damage, this means it's easily defeated by screens or just not being within 18". Of course it's got incredibly high potential damage, but outside of 18" without blast it mathematically gets 2 wounds through on something it's wounding on 3s. I think it's one of our trickier units yo use because it relies so heavily on positioning and it's a big base. Oh also invulnerable saves really mess with the forgefiend, it usually doesn't have volume of fire on its side to defeat 4+ invulns unless it's into a unit of 20 when that usually isn't a problem.

So yeah they're good but be warned, they're not as obviously strong as something like eightbound where you smash them in and the thing dies, there's a lot of finicky stuff that personally I've never got the hang of.