Hahaha well thank you for the permission! Allow my brain to vomit out some text for you...
When I say it's open to abuse I mean that there is potential for people to do some slimy things. If I wanted to I could sit 5 Slann behind a wall of self-regenerating undead and spend the whole game summoning Ripperdactyls and Carnosaurs. Or I could throw some Dwarf cannons into my Bloodbound army with a Lord of Change summoning more Lords of Change and a few Engines of the Gods to push for an extra turn. Or just an army consisting of multiple Archaons.
Really though to make a GOOD army it needs to be balanced. Synergy plays a huge part. I was recently trying to convince a friend of mine to get his Bretonnians up and running and there's a lot of benefit to using Bretonnian and Empire units as the "Free peoples" keyword is on both. Some Bretonnians also benefit from "Paladin" units being nearby, and some Stormcast units have the "Paladin" keyword. Careful planning could mean that a combined Empire/Bretonnian army with some Stormcast elements would be a force to be reckoned with - but just throwing down the most "powerful" unit en-masse would not be as effective and would miss out on rerolls, dice modifiers and bonus attacks generated from the benefits units grant one another.
Seraphon are the same - although our space lizard daemons are a tad racist and don't really benefit from being around other factions and like to keep their buffs to themselves. You can see it in almost any post that's along the lines of "I'm a new player and need help", "what makes unit X good?", "should I use unit X or unit Y?" it all comes down to what other units you're using, what your playstyle is and what your gaming environment is like.