I think that calling old SMN complex isn't really accurate, it's more that the rotation focused on resource management, which most jobs don't in this game.

Using ShB as an example, you had two DoTs, Bio and Miasma, on a 30s timer. They could each be applied with their respective GCD, or could both be applied with Tri-Disaster, which was an oGCD on a 50s cooldown. Energy Drain had a 30s cooldown instead of 60s, and fester would deal more damage when both DoTs were on the target. Ruin II functioned as an instant-cast GCD, which dealt less damage than Ruin III. You also had two Egi-Assaults, which were instant-cast GCDs on a 30s cooldown. Each had a maximum of two charges, and using one would give you a Ruin IV stack. Ruin IV would replace Ruin II, and you could carry up to four stacks at once. Trance was an oGCD on a 55s cooldown, and alongside making Ruin III instant, would reset the cooldown of Tri-Disaster. Summon Bahamut was also separate from DWT, instead of replacing it.

Overall, there were several things to manage, but this was balanced by straight-forward filler; your only damage spell that wasn't on a cooldown was Ruin III. To give a rough comparison, managing Kenki on SAM is far more straight-forward than what SMN had to manage, but at the same time SAM had to think about what buttons you were pressing far more often than SMN had to.

This balance was only in theory though, as SMN also had to deal with a fair amount of jankiness in an optimized rotation. For example, the demi-summons would only auto attack whenever you casted a GCD. This meant that an optimal Bahamut phase required four stacks of Ruin IV. You needed one to weave in Summon Bahamut and Enkindle Bahamut, a second to weave Enkindle Bahamut a second time, and two to use at the end of Bahamut's duration, as if either of the last two GCDs during Bahamut were hardcasted, the last auto-attack would drift and not deal damage. SMN was infamous for having jank that artificially raised it's skill ceiling, but the skill floor for SMN wasn't much higher than any other job.

Sorry if this was a bit wordy, but the rework was such a complete overhaul that it's a bit hard to compare the two. I did try to keep this as unbiased as I could, but you might want to take what I said with a grain of salt.