As said above, you need to use something else with Sigil and Doom Bolt if you want to clear the game without too much trouble.
I’ve cleared the whole game in Ultimate with my chaos/fire build and I can tell you that Ultimate has felt like hitting a giant brick wall with my head.
Chaos damage falls off very hard late game because of the way resistances work and because base chaos damage on skills is relatively low. Doom Bolt falls even harder because of its awful scaling and because it’s a single target skill with a cooldown that serves no purpose in ultimate where mobs are numerous and have a buttload of hp.
The changes to Thermite Mines and devotion were a blessing for my Pyro and now he can now kill Log in a minute with ease but if I had gone pure chaos using only SoC/DB like I had initially planned to, he may still be stuck in Act 3 Ult because you can’t count on itemization to fix anything ( hint : it sucks for chaos builds ).