As far as the Gossamid Archers, I'm not thinking they're too powerful. They are expensive and fragile, and I think that mortal wounds are kind of necessary for no rend and low rend units due to save-stacking. I think this unit has a very clear role, as a screen and charge-blocker that isn't pure cannon fodder. I actually think it sets a nice precedent for how to give units abilities like this linked to the generic command abilities that GW seems to set on emphasizing.
And as for the previous ongoing conversation, I definitely don't think that Seraphon are a weak army. We have awesome allegiance abilities and a few very powerful units. But I do think it's just a few. And I think that's where the problem a lot of people, myself included, have with Seraphon. Just because you can take a Skink and Salamander list and win events doesn't mean people shouldn't feel disappointed that you can't do the same with Saurus Warriors, Ripperdactyls and Carnosaurs. And yes, I know that people have won events with off-meta lists like the Ark of Sotek list and Koatl's Claw (which IIRC didn't really use a lot of Saurus). But people also win events with Gitz, BoC and Sylvaneth and no one's arguring that those armies are in fine shape. We have a *ton* of units and tools in the army, but some of those tools are razor-sharp while others are rusty and fragile.
I still consider myself a relative newcomer, having only been actively participating in the hobby for the last few years. However, I do think I've seen enough to know that there is big imbalance between how the army is seen and how it actually plays. Now, it's certainly understandable that some people enjoy the current playstyle. But I think it's also completely understandable that others wish there were more options available, given our very large model range.
I 100% think that Seraphon should have multiple playstyles in the book available other than shooting. I do think Saurus Warriors are much too weak to be used competitively. I think the same is true for a large portion of the army. I think it would be perfectly possible for GW to make our 3e book internally balanced much in the same way that Nurgle and Idoneth are. It seems that most of the units in Nurgle are at least viable, and Idoneth is a good example of how to balance a book that was previously known for being extremely spammy. Eels were given *slight* nerfs, not a heavy-handed nerfhammer that made them unusable. Meanwhile, pretty much everything else in the book was buffed to bring it up to a competitive level. Namarti, Sharks, and Leviadons are all at worst useable and at best even more competitive than Eels. But again, Eels aren't in the garbage bin. This is what I'm hoping for in our 3e book. Bring Salamanders to a more reasonable level (and bring their points down), but for the love of the Old Ones buff our other things so that they're all good!
I've heard people say a couple of times something more or less along the lines of "if all our units are good, we'll be way to OP." I disagree with this. First of all, you can't fit everything into one list. If you look at other 3e books where the majority of units are viable, it doesn't make them oppressively strong. Again, Idoneth is a good example of this. I'd say so is Nurgle. Stormcast would be, if not for the fact that they do have several blatantly OP units in addition to most of the rest of their book being at least decent. I would argue that if all our units were good, it wouldn't make us oppressive, as long as those units were oppressively OP. With the way Starborne and Coalesced are essentially different armies that use the same units, each army should have a very different playstyle, even more different than they are now. There should be room for the very technical, "dance around the board and win on objectives" playstyle, as well as the "push units forward and smash faces" playstyle. And also the "shoot stuff off the board" playstyle, and the "magic and summoning" playstyle, etc. And I think this is completely possible without making our army OP. The way I would accomplish this is probably not a method that some people would like, but I think it's easier to balance an army primarily through warscroll changes rather than allegiance abilities.
I think if most of your army strength comes from warscroll stats, with only minor buffs from allegiance abilities, similar to Stormcast and Idoneth, then you have an easier time making sure that you don't end up with massively OP combos. You also have more reliability, where you know what you unit can do and don't have to worry about setting up the perfect combo in order to have your unit perform at the level it should. Having units with good warscrolls won't result in us being "Stormcast 2.0" or make us lose our identity as a faction. It's the way things are going in 3e. Also, having units with good warscrolls would actually encourage mixed lists, which is another very common complaint. Seraphon is a spammy army because we rely so heavily on buffing one unit as much as possible and then sending it out to do as much as it can while those buffs are active. So the units that can take buffs the best while also having decent warscrolls are the ones that get used. But what if all our units had good warscrolls and the allegiance abilities didn't push you so hard into using one unit type? If you didn't have to focus all your efforts on just Skinks, just Dinosaurs, just Saurus, etc., then you could actually bring a mixed list without feeling like you were nerfing yourself.
Units' army roles are also better defined by their warscrolls. You want Skinks to be support and skirmishing units? Give them stats that reflect that role. Make sure that when buffed by allegiance abilities they don't become an unstoppable death blob. You want Saurus to be powerful fighters? Give them the stats they deserve. You want mortal wounds on 6s to hit, but not on a unit that can throw out 60 shots? Just give those mortals to Saurus on their warscrolls instead of as an allegiance ability. I know that this doesn't sit well with some people, but in my personal opinion it's the way Seraphon need to go in 3e. You can keep the same general feel of the army while also opening up playstyles other than spam the best unit or choke the board in fodder. Now granted, you could do similar things by having allegiance abilities that buffed specific units, but that's become less prevalent in 3e. Having better warscrolls *for the units that need it* wouldn't make us less of a "thinking man's army" for those who like that playstyle. If you want to run Skinks, Slann and Salamander running around and shooting stuff? You should be able to do that. You want to run Saurus Warriors that march forward and cut the enemy down? You should be able to do that. You want to run dinosaurs that overrun everything in their path? Well, you can already kind of do that, lol! But if you want to see mixed lists, this is something that's necessary. Will tournament players still spam the most mathematically efficient unit? Most likely. As people always say, Warhammer is a dice game. But taking as much of the randomness of the dice out of the equation is just what the most competitively minded people will always do.