I have seen it in tutorials and with projects of my own, that merge node almost always throws a warning about mis-match of attributes but never seems like a big deal because everything always works. so why the error or why does everything work with attribute mis-match, shouldn't something fail or not work as planned?

Every element of the geometry has to have a value of every attribute of its type that exists on geo

If you are merging 2 geos with different attribs, there is a mismatch

Merged geo will have all the attribs from all input geos,
therefore elements from inputs that were missing certain attribute will still have to have a specific value assigned, namely the default value defined for the attribute


For many cases you may be fine with default value, but you can notice issues in cases like N attribute, where geos with missing N will become black as the default value is {0,0,0} which is not a valid direction