Blissey and Chansey are great when they’re used properly. As gym defenders, their high HP works wonders and can keep a gym safe for a very long time. For some reason, people keep bringing them into Raids and it’s kind of a really bad thing. Sure, they can survive multiple attacks from a Legendary and that’s great. Just because they’re surviving, it doesn’t mean that they’re helping.
If you are trying to take down an Articuno with your Blissey or Chansey, that’s not helping your team, especially if there are already 4 or 5 Blisseys out there. The Pokemon might have great health, but their attack stats are absolutely garbage. They are so good at defending gyms because they take a lot of time to kill, not because they do a ton of damage. You’re much better off bringing in a lower HP Pokemon with significantly higher attack.
The faster the Legendary dies, the less health you will need. If you get a full team of people blasting down with heavy attacks as fast as possible, the Legendary won’t last nearly as long as it would against a full Blissey composition. Plan your battles accordingly and leave Blissey and Chansey at the gym. They weren’t meant for this kind of fight, and it’s time to change things up.