TLDR if you don't feel like reading the wall of text and my reasoning.
I understand that with materia you wanted to make sure there was a balance so that gear that was crafted would be useful and crafters would actually have a reason to craft items, but there is a huge problem with the logic that was used when trying to determine what could have materia added to it and what couldn't. The fact that Unique/Untradeable gear can not have materia added to them makes them completely worthless, and this negates about 80% of not only the content that was added in this patch, but a majority of the gear that was obtainable from previous content. This includes BOTH dungeons added just one patch ago and all the nms outside of world nms. Those things alone were the major selling points of 2 major patches yet it is completely pointless to do them now. I would also like to add that a majority of the gear that you get from settlements(outside of the sentinel's set) are near worthless as well because you can't add materia to them and the stats that they offer can easily be outdone with a single grade 1 or 2 materia.
My question to SE is, why do you do this? Is the +7 attack power and accuracy on a solid scale mail so game breaking that if you were to add materia to it it would become overpowered? I would honestly need to do further testing, but it seems to me like there is an incredibly low stat ceiling on stats as it is, and with buffs you are more than likely to reach that stat cap anyways. A good example of this would be the fact that I do not take any more or less damage when I pop a rampart against ifrit if I have protect II and defender on me with a bit of materia on my gear even though my defense goes from about 750 to over 900. Likewise could be said with my materiad attack gear. I added mercenarys slops and added a second materia to my hands giving me over 30 extra attack, yet my buffed attacks against ifrit has consistently stayed the same for each ifrit fight I do even with these additions. I really don't think the 5 defense on my templar's falchion, which I might add was the hardest and most sought after item in 1.18 should be made worse than a sword that you can buy for 100k just because you can't put materia on it. There is no reason that things that are Unique untradeable in beastmen strongholds should be outdone by these cheaply crafted items either.
I am an officer in one of the most active linkshells on my server and I can just see it on all of my linkshell members faces. Everyone is burnt out on ifrit runs for the most part, and there is nothing left to do other than level and farm the few worthwhile items in strongholds.
If you are worried about balance with these items just understand that the balance is in the fact that it would be incredibly difficult to get those double stacks on this gear because they are alot harder to obtain and can't be bought with money. The crafted gear could be for the people who don't have the time to invest in getting the best of the best of the best gear, while still being incredibly good and the unique untradeables that currently can't have materia added would be reserved for the people who play 40-50 hours a week and do want to go for the best. Again THIS DIFFERENCE WOULD BE INCREDIBLY MINOR and probably wouldn't even be noticeable for anyone that wasn't an elitest jerk such as myself recording what my damage output and intake is, and it would make darkhold and farming alot of the chests in strongholds that are currently worthless worth peoples time.