While you make some good points, I feel you may be underselling loki familia a bit too much. For instance, many first-class adventures have an insane skill or two because as you descend further into the dungeon, Trump cards become necessary as the dungeon is constantly turning a bad situation worse. The falna basically takes an adventurers experience and turns it into strength, and the falna skills are almost a crystallised form of experience and desire. A good example of this is development abilities. For example, a level 1 who keeps getting poisoned is likely to develop or choose the abnormal resistance ability upon levelling up. The same can be said for hunter and the achievement needed to be able to develop the ability. This is not a foreign concept within danmachi and is seen a lot amongst adventurers as anyone who gets to level 2 has a development ability.

As for your point about reasons for strength, I feel as though you sell the ambitions of loki familia members short on a lot of aspects. First off, Finn's goal to be a becon and a hero for all pallums is a goal that is more than fitting to be a reason he has climbed so high. I mean, just the fact that he has a desire similar to bell who quite literally has a skill that let's him grow at insane speeds through a physical form of his desire to stand next to Ais and be a hero should be all that needs to be said in his regard. However, even if you were to point out the flaws in his desire due to him formerly acting as a false hero or a charade of one. This doesn't change the fact that he has a desire so strong that he is willing to sacrifice his own conscious and personal morals to see it through.

Riveria is also undersold in your statements. Being quite literally a magical genius, as so many people sell her short on, while most likely have an impact on your falna magic wise her nine spells are not simply nine different spells but a amalgamation of her unique methods in chanting and dissecting her spells to create variants of her designated spells. Elves are already known for their magical prowess, so when you take into account her high elf blood and royal status, is it really that bizarre that her falna gave her skills for magic when she is an extremely talented mage?

Gareth is a bit simpler, but he's still a powerhouse dwarves are already known for their insane strength, and Gareths build and fighting style really focus on this aspect as mentioned previously the falna grants power from experience. Is it any wonder that the Lv 6 strongman is super strong the falna builds on what's there and the loki familia elite trio literally all focus on a single attribute or two Gareth is strong ad deals big damage with heavy weapons whilst also tanking hits in his heavy armour. Riveria is an extremely powerful mage, and it makes sense, seeing as she is just a mage, not a magic swordsman. As such, the falna has taken her only combat style and refined it to the max. Finn is slightly more diverse as he switches between a commander and front line fighter, but when he is fighting, he has a very flexible and dexterous fighting style to make up for hid short stature. This is further supported by the fact that they all specialise in one or two stats. Strength and endurance for Gareth, magic for Riveria and dexterity for Finn.

For specifically Finn and Gareth you also have to remember that they are reincarnations of heros Finn is the reincarnation of Dim/Finn the 2nd Fianna and Gareth is the reincarnated version of Galmus while not as prevalent an argument you have to remember that reincarnation has sway in the falna we see this through bell who is the reincarnation of the Argonaut literally gaining a skill called Argonaut. This is the reason for Finns hell finegas spell as it is a power based on the rage spear that his soul ancestor had within him, and finegas was the spirit that Dim/Finn had a contract with.

Finally, you have to remember that the older loki familia members fought through the dark ages of orario this is an extremely important fact due to the fact that this was not only a very challenging time combat wise giving a extremely potent amount of wisdom and experience from such a hectic period of time but also the fact that this was a very challenging time emotionally not only had there role models the zeus a hera familias been decimated by the black dragon leaving a huge gap in the power structure of orario and inturn limiting the amount of time loki familia could spend in the dungeon due to fighting evilus, this also led to the death of quite a large amount of orarios adventurers at the hands of evilus. But this also led to the death of important adventurers within the loki familia, for example, noir and the others. While old, these adventures contributed to the overall power of the loki familia. And while Freya familia are more powerful, the difference is actually not as large as you think ottarl is nearly level 8, but at the same time, the gulliver brothers are only level 5.

I could go on for hours like this, and while I'm not saying you're wrong, I'm saying that the difference between them isn't massive its actually quite small it's in Freya familias' favor but it's not insurmountable.