I don't particularly care for the story of the postgame so I will keep this focused on gameplay.
The boat maps are a hint of things to come, overly bulky enemies with the typical poor map design that pollutes the game. Things don't seem out of the ordinary until the last boat map, Archanea Seaway 6 is one of the worst maps in the game and the series as a whole. You are totally surrounded by enemies on a small boat as reinforcements pile in (Cantors do not appear in this map, so manner in which reinforcements spawn is unique to this map for no reason), and have to deal with the disgustingly bulky and hard hitting water dragons. It's very likely that you will have to use the revival shrines after clearing this map, just to get it over with.
The dungeon itself lacks any Mila Shrines, however, there's a trick to use bookmarks on the digital version of the game to save your progress without the shrines. The first half of the dungeon isn't very special, generic music and layouts just like the rest of the dungeons in the game. The second half is where things take a turn for the worse. For starters, enemy stats are inflated to some of the highest in the series on Hard Mode. One rounding enemies without the use of Double Lions, Hunter's Volley or other forms of effective damage or high crit is just about impossible. It's pointless to try and fight enemies at this point, avoiding them is for the best, but good luck doing that with the terrible controls in dungeons.
Lastly, we have Grima, who retains Dragonskin from FE13, but with no way to bypass it this time. Falchion does nothing to negate it, not even partially. Grima's high range and extreme damage output capable of 2HKO'ing even units with capped defenses and HP combined with his bulk and constant reinforcements make this battle a complete and total slog. Fitting way to describe the gameplay of this game as a whole, but I digress.
Overall, I hated the postgame. It was incredibly unfun, dull, sluggish and a complete waste of time. I only played it once and I plan to keep it that way.