First, the biggest deficit to the game is that there are 4 story routes but only 2 branch from the same route while blue and yellow require separate playthroughs for no reason. Having to play through the beginning section of and over is incredibly poor game design when there's no benefit for doing so outside the stuff from ng+. If they would change the maps for the beginning section and the story like they did in each of the 4 separate routes second halves it could have saved it. But as is I found it a slog trying to go through the yellow route after the red route when very very little was different between them even dialogue wise.
tldr requiring a minimum of 3 full playthroughs was bad game design.
Gameplay is next. The over all gameplay wasn't changed much since awakening/fates/echoes. They took away Awakenings and ups children mechanic and marriages, dragon pulses from fates and dungeons/towns from echoes but kept the multi classing, updated the food morale stamina system from echoes, brought back weapon usages, carried over the accessory equipment from echoes, swapped a few classes and removed some and added some different new ones and kept the rewind feature from echoes.
What was newer was the new crest system and special lord weapons and accessories system. The first is most units come with a special skill crest on them that can randomly activate during battle. Most are not very useful or important. But some have higher activation rates and are useful like say blocking counter attacks or healing from a portion of damage dealt.
Crest weapons and accessories are able to be used by any unit but will take 10 hp damage for ones that don't have the matching crest equipped after combat which is a major deterrent to using them on a unit without the crest. On ng+ you can purchase the crests of your units via crest stones in the renoun shop of the main characters bedroom office. This is very nice since not all units have crests and some can benefit from a crest weapon that would otherwise be useless to the character it is for. Like the swords Lysithea and Marianne can use with their matching crests but they are both mage leaning units. Or the axe Lorenz can equip which is better off on say Cyril. The arguably most important ones though are the magic staff accessories which give +2 and +3 magic attack range. The first one is only usable by Lysithea and Lorenz and the second by I believe Lin and Flayn since they each share a crest. Characters like Hubert, Dorothy etc all greatly benefit from them as well so it's nice on ng+ that you can equip them without the draw back if you use the crest stone.
Voice work was a large improvement from the 3ds titles where only the beginning line or a random voiced segment would be present but the rest of the dialogue wouldn't be. But not everything is voiced still so having the hodge podge voice work is kinda disappointing. It is somewhat understandable due to there being 4 different routes and branching dialogue in most of the in game conversations. But it feels like they could have consolidated some of it to a more practical use vs having branches of dialogue many players may never even encounter go to waste. The quality is there at least with very few bad performances and I was quite surprised that some of the characters share the same voice actors between routes.
Class system wise is lacking many of the staples from previous games and new editions. Though I did read that some classes were added in via the dlc such as the dark peg class but that being paid content isn't exactly a great solution. One of the returning classes Dancer is a weak one especially when the platoon skill stride can do it's job slightly better. And since you can only promote a unit to dancer once per playthrough via missable sidequest and it requires a stat check of charm to do so it hurts it further. If they had at least made the dancer function like the heron class from path of radiance and radiant dawn where it gave 1-4 units another turn that could have saved it. but as is I got very little utility from it compared to previous games where it had other benefits than giving a unit another turn.
Story routes they did it probably the most questionable way they could have handled it. In each story route, it essentially branches into an alternate timeline where whomever you didn't pick opposite of the character you did pick becomes the games primary antagonist and completely loses their minds basically. With the exception of the red route, this is especially apparent for a certain unit whose design is even drastically different between the 4 routes. Other changes happen like some units die in-between routes and are never seen again who were primary characters in another route among other things. Because the routes are that drastically different than each other it doesn't feel like they all came from the same story with the same characters.
Besides that, each story routes have checklists for what units you can and cannot use in them but there's little or no story justification for it outside the advisory roles 4 characters play to their respective lords. Seteth for Rhea, Dedue for Boar prince Dimitri, Hubert for Edelgard and Hilda for Claude and some minor units like Ashe for one route, Gilbert for a route and other units like Anna and Jeritza being exclusive specifically to red route only that were added in later as dlc. This part hurts the story in particular as almost none of the units are added into main story scenes for the most part and they are given little to no justification for joining other routes other than because they like the professor. You have some in cases like Lorenz and Lysithea joining the red route as they are close to empire territory and have connections to the empire. But some are outright conflicting like Petra being in any route other than the red route as that would lead to the destruction of her home country. They somewhat address that part by having her country attacked in the golden deer route during her paralogue special map and you save it from the empire vs in the red route it's someone else that attacks it. But that adds further plot holes on how they'd even get to Petras country located on an island off of enemy territory, how they would defend it and keep supply lines going among other things long term.
Whatever route you pick the majority of the characters can all be recruited during the first half of the game. But once you get to the second half some or most will leave and you can have the option to recruit some them back depending on the route. Some just leave and are gone or die off screen however. Hilda is also the sole exception where she can be recruited in a route other than her own under specific conditions during the second half of the game.