i have returned from testing, i just went and made what i think a common shieldball would be, it contained 25 aspis and 5 of each other unit from the shieldbot factory (minus snitch and convict), which totaled out to be 24,800 metal slightly more than a detriment
so, i limited myself to half that budget to find a solution that was mobile and doesnt use the previously mentioned "few and expensive" counters
my first plan was one that worked well enough i think, 100 gnats and 15 pyros (11,800 metal), send gnats in first, then have pyros moving in time to arrive as soon as shots start firing, the EMP from the gnats shut down a LOT of the shields pretty quick, and stopped a lot of return fire that would have hit the pyros, and the pyros able to do full AOE burn damage through the now shut down shields really gave the bots a nasty kick in their shielded pants
not total victory though, since a few survivors were left, 4 rogues, 3 outlaws, 2 thugs, 2 felons, and 2 vandals, though all at half health and with no shields
so while not total destruction, i think it works good enough, those few units are pretty scary still, but definitely not going to breach your late-game base, and since it's both faster and has more terrain access than a shieldball, they cant run away, keep in mind this was less than half the budget of the shieldball and i forgot to remove some of the caretakers i had up from earlier testing, so they also had the healing from 3 caretakers
im not usually one to just say "git gud" and call it a day, but i do agree with PLT_Godde, if you can almost completely destroy a shieldball with half the materials used to create the shieldball, and not using any of the counters that people dislike using (which btw are still good counters and are other options to help possibly make the metal input needed that much lower!), then i think you're just doing something wrong
EDIT:
i went even tougher and did 25 aspis and 10 of each other shield unit (again minus snitch and convict), now this one is very tough to deal with while still only using half their budget, i never did beat them, the main issues i run into on this version are the fact that felons and thugs have smaller shields than aspis, and the 300DPS aura while inside the shield from the outlaws, this greatly reduces the effectiveness of the main anti-shield tactic of mine, running inside and jumpscaring them, my best bet was to use a ton of pyros to run in like on the smaller test, and then hope that the enemy is either not using attack move so they dont retreat, or that they forget the pyro leaves a burning area behind on death, this resulted in about 25% - 50% of the ball destroyed depending on if they were running away, and if they did or did not walk over the after-fire
so yeah, i suppose if they have an army worth almost 35k in value, and you're stuck with only half that, you might just need to use missiles, cause at the end of the day this never was about "there's no counterplay to that strategy", it was about "i dont want to use the counterplay to that strategy"