John Hawkwood is as famous a mercenary boss as they come and his coat of arms is according to all regulations:
Federico da Montefeltro, a famous condottiere, same:
Black Army of Hungary, a known mercenary army, also according to the rules:
So yea. They also didn’t slap a random white skull on a blue and green striped background, just cause it seemed badass, but adhered to certain standards. I mean even the good ol’ pirate jolly roger is, after all, pretty much according to heraldic concepts, a white (metal) skull on a black (colour)background :)
Basic colours, basic partitions, basic patterns, even charges (main symbols) as diverse as they were, from fists to unicorns, swords to dragons, had certain rules of facing, positioning, etc.
Even medieval peasant uprisings saw the peasants adapt symbols, such as “Christ the just ruler” or certain patron saints, on their flags, but again, the colour of the flag and the charge (in that case the saint) were properly set.
In your current set, most are fine. But some are just really off, according to basic heraldry rules of how to combine colours (usually avoid colour on colour, or metal on metal), or positioning of the charge, or charge combinations. Even if you consider going “with the most “awesome” design they can come up with” it can still follow some of these very basic heraldry rules such as how to combine colours and where and how to place the charge.
It’s not a massive issue, nor one that would require you to rework a ton of things, but if you’re already at it, why not do it right?
I rather liked how (the otherwise relativelay lame game) Stronghold Kindoms allowed the player to play around with setting up their coat of arms with a bunch of heraldically correct elements.