When Frostferno was in the game, EC had a 5.5 link with +8 gems that was highly possible to use. You would take Avatar of Fire, get a double corrupted Frostferno and now you had a cool non traditional setup. It was reasonable to use sapphire at least due to cold to fire on Frostferno.

Frostferno is gone now though.

After a lot of PoB work, I am of the strong opinion that EC is better off without Sapphire/Topaz flasks. This, imho, basically makes it a broken gem.

1. It takes solid investment to get the flasks to be useable over a longer period. If you go pathfinder, you have very little synergy with EC. If you get the survival secrets jewel and invest on the tree, you can use it, but it just doesn't provide that much and your flasks are worse. You invest for fantasy basically.

2. You lose out on other flasks. Arguably the strongest flask is Dying Sun which gives 2 projectiles which is a major DPS bump as you go to 7x base damage from 5x base damage assuming you are using Greater Volley. This means the other two slots must be sapphire/topaz, BUT you can't actually use them or you don't get the damage bonus. You lose two flask slots for damage and honestly, a diamond and a cinderswallow is basically just as strong and you can still get suffixes.

Can I back up any of this? Kind of! Here are some theoretical PoBs. None of these are crazy expensive. These are end-game geared characters though.

https://pastebin.com/9EysbkYi Have a look. Compare it to other EC builds. Look at all those big shiny numbers.

Don't believe in charges with ralakesh's impatience? Eliminate ralakesh's impatience and all the charges and compare it. https://pastebin.com/Z3bcsydX still pretty good!

Those are pretty good characters on paper! I've played a much less geared version of those characters and killed normal Maven/EoW. It is a pretty good character, but the clear is bad and I can't fix it, so those are not recommendations and if you yoink those PoBs and hate your character then fair warning.

TLDR: Explosive concoction needs fixing but is playable if you just ignore most of the gem tags.