Is this the best support unit ever?

So here i was, doing the tempest trials, grinding points to get myself a shiny black armoured knight and i got myself a Sacred seal. This one happened to be Fortify Def 1. I looked through my units and found that Eirika was the best unit to equip this seal to since she is already a great buffer even without SI. So here is the build:

Weapon: Sieglinde
A slot: Fortress Def or Res
B slot: Swordbreaker 3
C slot: Hone Spd3
Assist: Rally Resistance
Special: Aegis or Pavise
Seal: Fortify Def 1

So here is the explanation. Sieglinde allows any nearby unit to gain 3 atk at the start of the turn. Hone Spd allows her to buff a nearby unit by giving them 4 extra speed. Rally Res buffs another 4. Fortify Def buffs by 2. Now she would be a monstrous support for blade tome users like Nino, Tharja and Male Robin since they will output 13 extra damage due to their buffs (albeit, rally resistance can only work for one at a time). This is also not including the buffs the blade tomes get from their own passives like darting blow or swift sparrow, which increases their total damage output by a huge amount, meaning they can one shot almost anything. This can really help when they double opponents and can be very helpful in most situations. An extra 26 potential damage is nothing to ignore. Not only that, but they can recieve a boost in all of their stats except HP. As for the special, since eirika is not supposed to be in combat, Pavise and Aegis help her to survive at least one hit on the battlefield. Fortress Res/Def will turn her into a more defensive unit, meaning she can stay on the battlefield longer and more easily deal with green units, while Swordbreaker allows her to more easily deal with reds like Ike or Ryoma and as Eirika's speed is higher, it is unlikely that anyone will double her, also increasing her survivability.

Thank you for reading through this whole explanation. Leave your thoughts down in the comments and tell me what you think. Don't be afraid to give your most honest answers, i will not be hurt by criticism.