When wired "backwards" small transformers (smaller than 3kVA singel phase) will exhibit worse than normal output voltage regulation and higher than normal inrush current. In either case these are not normally deal breakers just things you should be aware of.
The voltage regulation falls off because the "H" windings usually incorporate additional turns to compensate for the inherently poor regulation (well relatively so compared to bigger units) of small transformers. Now that the extra windings are on the output side they offer little help.
The inrush current can be up to 14x due to the location of the H and X windings in relation to the core. This is usually not a problem for transformers with less than 9A rated load on the X windings.
An isolation transformer is a separately derived system and must be grounded per article 250. It makes no difference if there is a grounded conductor on both the transformer primary and secondary, you must follow correct grounding and bonding practices.
