Nexon is so spiteful and malicious towards its Reboot player base that they're willing to sacrifice money, the only thing they care about, from cube sale profits in regular servers to discourage Reboot players. They'll either ignore this post entirely or lie about the reason they're doing it like they do with everything else. They're so obsessed with spiting their players at every opportunity that they even ban and lie about ban reasons for people selling frenzy service. Literal psychopaths.
This post is excellent and I agree with it in its entirety. Double thanks for writing it in Korean. Guaranteed Nexon will ignore it though. 0% chance they'll respond.
Chris is absolutely right here. The issues, in order of importance, are:
1. Familiar slot expansion coupons MUST be added to reboot 2. Red familiar cards MUST be obtainable in reboot 3. Familiar Breakthrough Cards MUST be obtainable in reboot 4. Familiar drop rate should be increased.
Users in reboot who have over 100 familiars are presented with a dilemma: Undo our hard work and get rid of the familiars we like so much to make room for new ones OR only ever be able to add 20 more familiars to our collection. Many users value familiars for the collecting aspect and without familiar slot expansion coupons the system will miss the point.
The exclusion of red familiar cards and familiar breakthrough cards also puts meta players in a difficult position. Without red familiar cards we'll have to restart from scratch if we don't get the unique potential we're looking for. The problem will get even worse as we accumulate more familiars with good unique potentials since the number of potentials we'll want will decrease. Furthermore, we'll be working much harder for much weaker potentials because we won't have the familiar breakthrough cards necessary to obtain top tier potentials.
Lastly, the drop rate of high level familiars should be increased. With the way the current system is now, players will be forced to enhance only the familiars with the highest drop rates (shroom, snail, blue snail, red snail, slime, orange mushroom, etc). It would be extremely unsatisfying to have the weakest and lowest level familiars be the best because they're the ones that can obtain the best potentials. I'd hate to see the first reboot black mage clear done with 6 of each color snail. It would look much better with Limina familiars.
I'm yet another player who spends a considerable amount of time hunting familiars and monster cards (762/752 monster cards, 310 familiars, currently level 259 in Reboot).
Right now the only way to know which monsters drop which items to look through the crusader codex. Monster cards that you've collected in the codex will display, among other useful information, the drops for those monsters. For certain items with extremely low drop rates such as the Daiwa Sword from Leader A, world boss familiars like Lyka, and instanced boss familiars like Lord Skeleton, this is the only way to verify the existence of their drops with absolute certainty.
Will we see a new source of truth for drops to replace the codex?
Secondly, in JMS there are many more monsters that drop familiars. Currently in GMS it is impossible to obtain familiars from Arcane River through drops. Familiars that do not drop from monsters MUST be obtained through the cash shop and are only obtainable on non-Reboot servers.
Will all the monsters that drop familiars in JMS also drop familiars in GMS?
and 8 others. 
