From my experience, the eldritch summons were worse than summoning undead (edit: at lower levels). You'd get multiple undead at relatively early levels, and even without necromancy focuses, they were far less likely to get overwhelmed and killed. When my tier 1 and 2 eldtrich summons get stomped by derro, its not worth burning a feat over early on. The mid-tier ones don't fare much better when compared to create undead. The epic versions are the only ones i've been able to consistently use without them dying frequently.
I play a Night Hag pacted, so I can't comment on the other pact types. The spell changes are...kinda wierd? Most pacted types can empower their blasts and have a good spell spread with 2 spell focus groups; hags can't.
The added spells for hag pacted are a mix of necromancy, enchantment, illusion and transmutation. Hag pacted characters pretty much need enchantment spell focuses to empower their blasts, which puts them in a wierd spot trying to pick out their second spell school focus as their spells are haphazardly spread out between 3 other schools (illusion, necromancy, and some transmutation)
A lot of the spells fit thematically with hags, but its difficult to effectively use them when warlocks realistically can only get 2 spelll schools to focus in. Nightmare is very powerful, but only works on sleeping targets. And if you fail to kill something with nightmare and it wakes up, it will become immune to sleep This makes nightmare a liability if you can't kill enemies in time, as sleep is your main form of crowd control and way of boosting your spell dcs along with mindfog.
As a counterpoint to the above about unseelie, the -damage- types of unseelie might be objectively worse (positive being the best, with cold/acid being commonly resisted by certain enemies/biomes), but almost all their spells can be snugly covered by transmutation and enchantment spell focuses and they have a very powerful secondary effect on their blasts that do not require additional feat investments to be effective. Since their spells are more "focused" in terms of spell school types, they have access to a far better selection of effective spells as well (confusion, imp invisibilty, haste, mindfog, spike growth, entangle, slow and dominate person are all very powerful when used properly).
I've seen people comment in-game that almost all the pacted types they've seen are unseelie; theres a reason for that, at a glance. Aside from the fathomless pact type, everything else looks objectively worse by varying degrees. Either because their spells are spread out all over the place in terms of spell types, or their blast effects have a less useful effect that may or may not require 2 spell focus feats to fully make use of. Or both.