Not sure it is actually that much of an unpopular opinion really It is however the reason that I now have 9 armies!
Iirc in a recent US event 4 armies had FoN allies (despite no one taking a FoN army) and understand that those were heartpiercers/unicorn or HP/wiltfather; 3 lots of ogre allies - boomers or siege breakers.
An allies matrix, or a far more restrictive list of what you can take would restrict much of the abuse, while still leaving things flexible for fluff builds.
Potential example is the mythical section of the KoW Historical’s book - there are some general archetype units (zombies, ogres, a large cav option, a middling fkying monster etc) but nothing really good?
The other option, while keeping them, is maybe a +10% cost increase?