I dont think making boidmachines a single piece artillery will break game balance.I am not saying to make it op,i am just saying to make it like nuke cannon or what ever you can think as balanced.In generals china can built lots of nuke cannon and has access to most powerful tanks.
in mental omega china has centurian artillery and powerful tanks.Now that didnt break game balance right?
As for Harbinger coronia can get an exclusive airfield for this unit.Make it less durable but controllable.Its not going to revamp their tech tree.Just remove the tower and add an airfield.
And for eureka i suggest she gets two types of attack.one for vehicles and one for infantry.deploying her changes her attack style.her green wave is lethal to vehicles and red wave is lethal to infantry.Also Pteranodons and eureka unlock at tier 3 so she cant be substitute for Pteranodon's.Removing her speed buff will be good too.She cant get speed buff without wind turbines and those can be easily destroyed wasting money.She should be a heavy unit who moves slowly deals heavy damage and have high health.Making her hit and run unit seems odd.
Like the other people who posted here, i dont see the point in making every Foehn subfaction having a directly in battle controllable epic unit. Harb and Boid are fine as is, both are really powerful yet not just another epic unit like the centurion that you run on the frontlines. They are arguably more interesting than Centy (ok, maybe not the harb). Neitehr Boid nor Harb need changes imo.
Boid hits vehicles really hard with infinite range and manual firing, too bad elevation differences screw up the targeting, thats the only issue i have with the unit. Harb gives all players a big timer telling em when the thing arrives, goes around like 8 times wiping out everything below itself if it doesnt get shot down. Its hella tanky to boot, but each call in costs 3k.
Coronia is the subversive of the Foehn subfactions, they are meant to be hit n run and focus mobility over armor. What you baiscally suggest here is a ground based Uragan for Eurekas role which simply does not fit in with Coronias doctrine.
Though to be fair, neither does Tarchia, even with spinblade boost the thing is slow af. It also the most tanky out of the arty units, not considering epic units. Still hoping for some change there, maybe burst of 2 instead of three shots at 2/3 of the current lock on time, with maybe less damage per shot to balance out the much higher reliability of it?
The idea of making Eureka a mobile spinblade sounds intriguing, esp given how all Coronia unique units are either flying or spinblade boostable. Cant deny, atm Eureka is the most disappionting out of all the heroes, and she did already recive quite a few buffs since 3.3.0. This is more because the other heroes are so overpowered rather than Eureka being weak however.