So my opinion on this is that the design it's not good enough not even interesting. The items don't have to be broken or too strong to be worthy. They need to add something to your playstyle, like bomb-chu (this was a really cool and interesting design and add something to the playstyle and not broken at all).
About the new panzers It could be an option for people that have low/mid tier gear, but as stated before, they aren't interesting enough and the rates don't reflect the usefullness of the item. I mean, if i was starting playing now and needed gear i wouldn't even think about investing time and effort to get any of those items, because they are really hard to get and i could simply replace them with whole year obtainable items and way cheaper in time and money (1/200 ish in a final boss, its damn difficult).
Another thing that you must note, is that panzer faust drops with low stats. I think that it's programmed to be like that. This means that your arrest/frozen faust will be expensive for you, when you have better options that lipelis stated well and way cheaper.
I don't agree with them being multi-target, that would be a bit broken and i don't like broken weapons. I think the problem is more with the concept/design and their droprates. They should be moved to another Mob other than a final boss, with a better rate (like lindcray for example. And lindcray is a god tier weapon and has a fair rate, fair stats when dropping, etc).
About Ultima Bringer, i don't know what to say. The concept, the usefullness and the rate are in the same line as the previous ones. Bringer's rifle its a way better option in terms of raw power and price and also obtainable during the whole year and easier. You don't miss that megid boost, because it's not exclusive at all, since some other weapons have this. Also, bringer's rifle it's usable in more classes.
I really don't have opinion on making the demons pierce, but i think it could work and not be broken.
In my opinion, the weapons should stay as they are, but where they drop and the rate at which they drop, should be changed in the future events.
EDIT: Also forgot to add: You could simply use a boomerang or a s-rank arrest j-cutter with 0 hit SNS glitching, and having way better results than using a Arrest Faust (not to talk that you probably have to add hit on it). The same thing can be done with a blizzard s-rank j-cutter instead of a Frozen Faust.