Yes, they’re fun to play. I enjoy playing the EX Block Format (a format using all cards from generation 3) on TCGONE because of the wide variety of playable decks. In fact, there are probably over a dozen tier 1 decks in that format, including both popular decks from the 2005, 2006 and 2007 seasons and all-new decks that could never have succeeded—or in some cases even have existed—in any worlds season. (For example, Japanese players broke Delcatty from Ruby and Sapphire wide open when they paired it with Surprise! Time Machine to reuse Energy Draw multiple times on the same Pokémon and Electrode ex to attach those Energy.)
However, the more viable decks there are in a format, the harder it is to pick a deck that gives you the best chance of winning. Every deck has strong and poor matchups, and when the top players invest $70 to go a tournament, they want to maximize their odds of winning and claiming champion points, as well as a more financial return on their investment. (After all, you can sell your deck after a bad run, but you can’t sell your ticket stub.) When one deck dominates the format, you can focus on beating that one deck, and if you can beat most of the other decks even semi-consistently, your odds of succeeding are strong.
Here is an article Grant Manley wrote during the Silver Tempest era.
https://www.pokebeach.com/2023/02/destruction-or-bust
Remember how much his ticket cost him, and perhaps you’ll understand why he was upset as he was about everyone teching against a deck that comprised 1% of the field.
In any case, Grant’s 8-12 run was carried by a 7-0 Lugia VSTAR matchup, and would have been vastly improved had he run into fewer tech-heavy Lost Box decks. Other players during the Silver Tempest era made top 8 with Control, Flaaffy box, and even Durant Mill. Lugia would then tech against those decks for the next event and go unrivaled, but with each event that rogue decks went unnoticed, less people would tech against them, until they would eventually make top 8 again.
Now there are many different play styles of deck, and it is hard for any one deck to compete with all of them. Let’s say your deck has a favorable matchup against Lost Box and Iron Valiant but has a poor matchup against Charizard and Gardevoir. You face several Lost Box, Iron Valiant, Snorlax Stall and Chien-Pao decks, meet only one Charizard deck, and go 7-9 in Swiss. You make top 32 only to find that eight Gardevoir players also made it. With lower stakes, having to battle through as many as four Gardevoir decks in a row may not seem like a big deal, since you’ve played against five other types of decks and are having the time of your life, but at a tournament, it can seem like the end of the world, as the chances of making good on your $70 investment are looking more and more bleak.
Incidentally, there is an online tournament being held for the EX Block format, and I am definitely feeling the pressure of not knowing what deck to play in a format with over a dozen top decks. Not that I will have to have to pay an entry fee or anything, but there is some prizing on the line, and it would feel great to win something. I haven’t played Base-Fossil for very long, but the smaller number of top decks there frees me up to focus on learning the format. Sure, Wigglytuff/Magmar has to have some bad matchups, but with only three other top decks (Lickitung Stall, Moltres Lickitung, and the go-big-or-go-home Arcanine/Electrode), I can learn those matchups without worrying as much about other viable decks like Rain Dance which I already have an answer to. (As for Prop 15/3, some days I wonder if Jason Klaczynski is the Prop 15/3 meta, as most players continue to build their own decks from scratch well after Jason identified the current top 3 decks. To be fair, there has only been one modern Prop 15/3 tournament with over 8 players, the 11 player Woodfield Mall tournament.)