Now the delve changes have been rolled out for a week, so I think it's safe to make a conclusion now.
I am not a big fan of the delve mob frequency decrease. Infernal is the exception here because the old ones were too generic to make a difference. For the rest of the delve mods however, even with individual mob buffs, the experience still feels like a downgrade.
More frequent delve mobs are good because it can consistently influnce your playstyle.
Having for example, a dread spawning every elite killed feels very different compared to having it only spawn 60% of the time, even given the dread powerups. Before the frequency change it felt like I have to actively adjust my build/playstyle to take the added mobs well, but post-change the lack of consistency turned most mods into "stuff happens here and there I guess". The rarer the delve mob is the more apparent the problem becomes, no matter how powerful the said mob is.
Consider a more extreme example: If every mob killed spawns a spectre, I would likely maintain some "second strike ability" in my build, but if it only spawns with a 1% chance, there won't be a need for any special measures save for "panic button to kill the elite mobs that sometimes pop up".
Also double dread is funny and good.