So, Jamz, I think you're misunderstanding something.
Reading your reasoning, it seems you're expecting the Adjustment to be able to reference the related spell to see requirements, and to validate changes (esp. in your example case of the Goliath druid and the class feature).
That's not true.
The adjustment is a separate thing. The only link is that someone named it for the spell as a spell adjustment. The "Enlarge Person" spell adjustment could be renamed "Make Creature 1 Size Bigger" and would still work as it does now... it would just be harder for someone casting Enlarge Person in play to know that's the right one to apply.
Even if there was a straightforward way to trace from the adjustment back to the spell, the "Targets" entry is just text. It would take a lot of text parsing to pick apart what the "Targets" entry means and get it down to a Type or SubType requirement. So asking the adjustment to check for Types based on the spell entry is ... not practical, really, even if it were possible.
Any validation has to be coded on the Adjustment based on the author's knowledge of how it works.. and re-coded over and over for every new exception if it is going to handle them. That is why it is often easier not to include them... because GMs making house rules will complain that they cannot get rid of the warnings.