Ferro-Lamellor is a small increase for a big cost overall in terms of slots and weight. Id prefer to use Ferro-Fibrous to cut weight down for more equipment(and then only on light-mediums), Stealth Armor for long-distance tactical units, or Hardened Armor for sheer durability. I does not "WOW" me.
FL isn't particularly heavy, especially when compared it to Hardened of all things (14 points per ton vs 8 ). It also really undersells both the universal applicability of its mitigation and the lack of painful or potentially catastrophic downsides. For instance, Hardened's -1 run MP means that it's a harder to hit your maximum possible TMM unless you overbuild your engine (such as a 5/8 dropping to 5/7 with Hardened, or 7/11 dropping to 7/10). Statistically, that's going to result in a few more hits landing and offsetting the armor increase. And Hardened doesn't actually
mitigate damage, so taking a AC20 hit is still going to result in a PSR, which has that increased difficulty.
Ferro Lamellor actually negates damage. That AC20 is going to be doing only 16 damage, so no PSR. It also continues to apply its bonus so long as a single point of armor remains. Fire that AC20 at a torso with 1 point of FL is going to result in only 15 points of internal damage.
Given faction availability, yeah, Hardened Armor designs are going to be more prolific than Ferro Lamellor, but that's a case of taking hardened lemons and making lemonade. Long term, I would expect Ferro Lamellor to become much more common over Hardened; and in fact, would expect Hardened to be superseded in favor of Ballistic Reinforced.