This is the game I accidentally dropped Etrian for. I found a physical copy of Birthright at a retail store for full, not-marked-up price and bought it because that's not an easy find. Even used copies of this game are generally sold above MSRP (though not as badly marked-up as Awakening) and while I could have bought it digitally, my 3DS memory is limited and I kinda like owning Fire Emblem games.
This game is worse than Awakening. It is a game about quantity over quality, and has a staggering number of units for you to control, level up, specialize skills for, marry off, and pass on skills and stats to their kids. There are a LOT of systems in this game (these games?) and playing with them can fill a lot of hours.
That said, the main quest for each of the three games that make up Fates (Birthright, Conquest, and Revelations) is maybe not that long when each game is 40$, and the quality of the writing and of the plot itself is poor. But after buying one game at full price, the other two are discounted to 20$ a piece. Each game's main quest is as long as the main quests of the others (Revelations might be shorter, I'm not sure) making them a steal at 20$, more than compensating for "overpaying" for the first game you purchased.
And if you want to engage with all the systems more than the main quest requires, even one game can last you many, many more hours than that main story takes to complete... for Birthright, at least. I know Conquest is more restrictive in how much it'll let you do between missions, but I had some DLC (all of which is accessible in all three games, and after purchasing once - thank god) that mitigated that. But you can spend dozens, if not hundreds of hours grinding levels, grinding money, learning specific skills to create broken character builds, making the characters' relationships stronger, deciding who will marry who, upgrading weapons, managing your personal castle, visiting other players' castles to trade with or fight against, playing on harder difficulties...
There's a ton of stuff to occupy your time with in Fates. Unfortunately, the end result isn't the most cohesive thing in the world, and there are enough systems that trying to pick up playing from an old save after being away from it for a long time is extremely difficult. And in this huge pool of "content" available to you, none of it is really exceptional. Almost every previous Fire Emblem game (released in the US, at least) has done a few things significantly better than Fates does, though which aspects are better and which are not vary from game to game. The games released prior to Awakening all have significantly better characters and writing. Even Awakening, which is written in a similar style and (like Fates) leans heavily on anime tropes, tells a more engaging story than Fates and uses its cliched character archetypes to far better effect.
I say all this, but I invested about 200 hours between the three games anyway. There is something to be said for having that huge quantity of content, and all of it's at least serviceable, even good at times. The trope-ridden writing and characters are weak, but that style of character writing makes it easier to use as mindless escapism. There's nothing underneath the surface you have to search for; every character behaves exactly as expected, and when you turn your brain off, they elicit the occasional smile as they say something nice or show affection for another character. It is occasionally even genuinely endearing, too. It's just a game that was perfectly designed to let me slip away into.
And in spite of my complaints, I do think the game is good overall. But I loved Awakening quite a bit. I spent a bunch of money on an imported Lucina Amiibo I saw at NY Comic Con because the US ones immediately sold out in all the retail stores I frequented. My affinity for Awakening runs deep. And this game didn't live up to that. It was good, and it filled the void I needed it to fill for quite a while, but... I had higher expectations than that, you know?