UC runs with the MW tribute yield 2000… and I’ve gotten more than that from hordes. NMD isn’t as lucrative but I still run them to mix it up.
Souls drop in less numbers. A T4 Belial kill was giving me like 40-60 crystals… souls like 7-8. That’s how you end up short on souls. No problems with any of the other mats but when you are attempting to triple crit MW all your gear… the 1-8 aren’t all that expensive… but 9-12 are… and if you miss it and have to reset… well… there you go.
The materials and chaos chests are a good source of souls… as are the rep chests. Other than those you are looking at 7-8 from boss kills… blood maiden… helltide chests… etc…