Pressure fermenting and temperature control? And water profiling?

[MashBag]

I suspect there is quite a large margin for the error though because many kits recommend adding brewing sugar or beer enhancer, I would think beer enhancer has more than just sugar (I might be wrong)

Beer enhancer most likely a blend of sugar (corn or inverted prolly) and some spray malt. You add it instead of sugar.

Not really anything to do with the water you start with.

 

[Sadfield]

The only starting point imo is the water for that style surely?

Correct. The starting water is the strike water the brewer prepares for mashing, if you're doing a kit that isn't you. When adding water to a kit, you're just rehydrating wort. Adding minerals on top of minerals.

 

[MashBag]

When adding water to a kit, you're just rehydrating wort.

To drill down further, you are adding back what the vac evaporator removed.

 

[Sadfield]

To drill down further, you are adding back what the vac evaporator removed.

A bit like boiling a kettle dry, where H2O and volatiles like chlorine are removed, but mineral solids are left behind in the kettle.