Between Jellicent, Heatran, and Roserade, you cover quite a few big threats, such as the ones you mentioned - Reuniclus and Volcarona. Reuniclus may still be troublesome, but with special defensive investment, Taunt, possibly Shadow Ball + Cursed Body as an ability, it can handle Reuniclus decently.
One huge problem teams with Roserade usually have (and a huge problem I had when I was using my team with Roserade), are Dragons like Haxorus, which is why I used Skarmory since it's a good catch-check to them. Though, Jelly + Skarm gives a nasty Electric weakness, which is why I went with Gastrodon for a resist, and Gengar as the spinblocker, but that other alternative is still viable.