Given that we now have the astropy.units framework, should there be at least a warning if columns with unequal units are merged ?
In simple operations add, multiply etc. the units checking is very strict, so as a user I could naively expect that values are transformed automatically if I join to tables that have fluxes as e.g. ergs/s and W/s. 
I don't think that needs to be implemented right now, but I would like some warning output that is it not (as that's easy to overlook in the documentation).