Shadow Dragon was just a step backward after PoR and RD accomplished so much. Shadow Dragon uses generic sprites for every character except for Marth and Medeus and Tiki, and eliminated support conversations, effectively making all the other members of Marth's army just another face in the crowd; PoR and RD made sure to make individual character sprites with little similarities for every playable character and major antagonist, except for Tormod, who had the Fire Sage model.
PoR and RD had multiple scenarios and objectives for the cast to accomplish, like Ike and the mercenaries crossing a bridge and distracting the Daein troops while Mist and Rolf took Elincia across another, far away bridge, or Ike and his growing Crimean army assigned to do tasks for the Begnion senate, include relentlessly chasing a corrupt senator through a dead forest until he was struck down, or burning the rations at an enemy base camp so Begnion would be forced to call back General Zelgius, allowing the Gallians to seize an advantageous piece of the terrain in the war.
All Shadow Dragon did was place Marth and his merry band in a new location, and have Marth seize the fort or city, and rinse and repeat, and told the story in between the chapters. Very little sidequests in the chapters that would also push the story along like the Begnion missions did. There was no variety in the game chapters aside from the mission to talk with the Manekete Princess.
Really, I think if PoR and RD had not existed, Shadow Dragon would have seen more favorable criticism. PoR and RD expanded a lot of things in the FE style of gameplay; Shadow Dragon effectively took a few steps backward, and having PoR and RD to compare it to makes the negatives stand out more, remake or not.