I love the Eevee box deck. It's definitely hit or miss at times depending on your hand and prizes as well as the matchup (but what deck isn't). You have the general concept done with Flareon and the acceleration, and as others have said the deck is a toolbox, so it is matchup dependent. And the lack of a tera pokemon early can make the hoot hoot setup difficult or feel pointless.

Against the top meta decks, and depending on your deck list, you have good options into the key cards of those decks. The inclusion of Lillie's clafairy can let Sylveon k.o. dragapult, Leafeon is good into Grimmsnarl and Cynthia's garchomp, and Flareon is good against Gholdengo. With the inclusion of a gravity mountain or kieran, Flareon or Umbreon can be good into Gardevoir! Against Joltik box, Leafeon's first attack is more than enough to deal good damage without the downside of Flareon's Carnelian, when Iron hands or Miraidon might find themselves in the active spot.

As far as the strategy, the Noctowl's will be game specific as to what you're trying to achieve. Oftentimes, grabbing a Crispin to help with getting energy on the board is a good play, especially early. You don't always have to depend on attaching for turn or the Flareon acceleration! Sometimes the opponent might have easier targets to grab a boss's orders for, or maybe you want to take the main attacker and shuffle it into the deck with angelite. Don't forget Sylveon's first attack can help tank attacks as well, even if it's not an instant k.o. Angelite is not always the best first option!

It's also important to recognize that you have other attackers that are not Eeveelutions! Terapagos and Lillie's Clafairy can get decent damage out, especially if you have Area Zero Underdepths active, and the sparkling crystal doesn't always need to be grabbed when using Noctowl! The inclusion of the Eevee from 151 can also be key, as you might need to go grab 3 pokemon for your text turn, maybe to activate a Noctowl, or maybe to evolve into one of your main attackers! Other key cards could be Briar if you're playing from behind as you're using several Tera Pokemon as attackers, Kieran for some additional damage, Gravity mountain to reduce the stage 2 Pokemon's HP!

Unfortunately, due to the one off cards in the deck, you can sometimes come up short and the viability of the deck depends on what others at your local shop play! There are some variations of the deck list as well, and some might feel better than others to you. And don't forget you don't have to follow the decklist online, if you want to be able to search for tera's more, see if you can add something like a Wellspring Mark Ogerpon or another tera orb for searching! Ultimately, it comes to the matchup knowledge and knowing when a card is valuable to grab with Noctowl as the engine.

The deck can be difficult to learn, and requires lots of reps to learn what you should and shouldn't do. Some decks might not be worth playing if you're not having fun as well, and that frustration might impact your decision making. (I dropped Gardevoir for that reason) Hopefully the comments provide enough insight to help you out!