Pre-rotation, Stage 2 decks do have some Pokémon setup support and you still need time (1-2 turns) to ideally setup them up. You also need X number of support cards to facilitate the setup (e.g., Rare Candy). So, in most cases, Stage 2 decks are inherently "slow." Further, you may incorporate some Basic Pokémon to serve as an initial attacker (e.g., Buzzwole GX) to apply some early damage and pressure in order to buy you some time to setup your main, Stage 2 Pokémon line(s). Again, this tactic could be helpful, but you could also lose a few prizes while you're still in setup mode. It's not unlikely that you could be playing from behind in the prize count (1-2 prizes?) by turn 4... However, some Stage 2 based decks (e.g., Vikavolt-Rayquaza GX) have great "comeback" capability. Finding a good Basic Pokémon to pair with your Stage 2 Pokémon is very important; the deck needs to have a "well balanced" set of support (Pokémon search, card-draw support, Pokémon line setup, energy, etc.); and the deck needs to be as "fast" (i.e., least amount of turns needed before you can attack) as possible in order to compete well too.
Post-rotation, a lot of Pokémon setup cards (Rare Candy, Ultra Ball, etc.) are unavailable; so, this makes running a Stage 2 deck competitively even more difficult. But, I also believe in playing decks I like and accepting perhaps the higher risks associated with that deck. So, I suggest you play the deck you like even if it is a Stage 2 and just prepare to compete against other faster decks as best as you can.
My overall stance now and post-rotation is that I don't like any Stage 2 based decks due to a variety of concerns including those mentioned above.