"Before we go any further, I'd like to remind you that cybrans have mermaids, which will effectively make your subs useless unless you have other ships that can deal with them."
This is true! I was trying to avoid a entire naval discussion, but I guess there's really no point haha.
"In any realistic situation, there would be a mix of units."
True! I guess my point is that a mix of units does not really solve the issue for sera in this case. Sera destroyers can only fight cybran ones by starting submerged and then coming up when they are in ranged and microing, but with the stealth advantage as well as the mermaid, it pushes the t2 naval combat quite strongly in the cybrans favor. When their t1 is already the best, its peculiar that the Seraphim pay off for surviving till t3 is a sub that is not mass efficient vs that faction.
"If you are in a situation where you have t3 hq and the enemy is spamming t2 subs, you could just make battleships instead, groundfire them, and then proceed to bombard his base."
This is kinda true, but it requires micro on one side with no micro on the other, as well as dodges the point a bit. sera and cybran battleships are pretty close all things considered, but across the entire rest of the navy the only unit v unit sera win (controlling for mass) is with very small numbers of destroyers when the cybran player does not build a mermaid. which is silly if the sera have to wait an entire tier to get a unit just to find its not mass efficient.
I am not even sure of the point you are trying to make here.
" Sera t3 subs are pretty strong as support units from the back of your navy, they shouldn't be your main force. "
This is also a really good point, comparisons in a vacuum are meaningless, and the t3 is fundamentally a strong unit. However this does not really excuse the fact that the cybran navy at t2 is sufficient to beat the sera navy at t3.
"I'm also not sure why you are saying that they are very strong against UEF? Are you forgetting about their bulwarks and that the coopers are pretty efficient at their job?"
Nope! but sera destroyers can micro in to kill coopers and then dive back under water, and sera t3 subs still do their intended job against uef navies, because jamming on surface boats is much easier to beat than stealth on underwater units.
"I'm not even getting here into other options, such as air (torps, all faction) or harms (which costs the same as your subs, outranges it, and also has stealth-but it doesn't move)."
Harms are stationary, which allows cruisers, bs, or other units to ground fire it. barracudas can keep moving (at a very fast 6.5). This means to even have a chance of ground firing them you need constant vision, which I think I have shown the Sera dont have access to. Harms only need to be spotted once.
t2 torps are really really cost effective vs subs. but against a full navy, as well as the enemy airforce, they are not nearly as viable, especially after asfs role out. If the cybran players spams inties and subs, the sera plays has to spam subs or destroyers to keep in the naval game, enough inties to take the cybran ones down with losing naval control (cruisers will have to move up into sub range to protect torpedo bombers from inties), and then have the eco for torp bombers.
This kinda makes me re-assured in my point: the sera navy isn't overly bad, nor the cybran navy overly good, but the seraphim lack a cost efficient solution to a realistic cyrban navy push at any stage of the game, due to their reliance on range play and lack of a viable vision tool.