Well, I think you're wrong on that, but it's completely subjective, so we can't really argue any more than we already have. I guess we've been skirting around an actual list, so for the record, here's what you can summon:
- 2 dryad, CR 1, can talk to plants/animals and cast barkskin, and can each charm some humanoid with a decent save
So pretty decent.
- 1 seahag. CR 2, has a death glare that's pretty cool, but takes 2 easy saving throws and will kill your party and not the monsters you're fighting
I disagree. If you have warning, you can avert your gaze. You can have your party prepare for the eventuality of a hag appearing, but your foes won't be prepared. You can also command the hag to put up her Illusory Appearance (and get frightened another way, probably through team work), or even throw up your own illusion of a wall/box between the hag and the party (a cantrip - minor illusion). Bottom line, the hag can be useful if you are prepared for that option, and can outright kill foes with that gaze. Not useless at all. And a lot easier to prepare for with this ruling being that the summons is connected to the particular setting and situation. If you are in a swamp, the odds of a sea hag go way up and you're more prepared for that outcome.
- 4 satyrs, CR 1/2, have magic resistance but otherwise suck, I'd rather have 8 wolves
31 hit points versus 11? Higher AC, higher attack bonus, higher damage bonus, ranged attack, can wield weapons and wear armor, etc..come on, the satyr is a pretty good summon.
- 8 sprites, CR 1/4, can poison at range and tell you alignment of things, but 2 HP and otherwise useless
And INVISIBLE with a +8 stealth. Fly over there, invisible and silent, and tell me what creatures you see and their alignment - yeah, you want that sometimes. Even in combat, it's not a wasted spell (though it might not be the desired effect) to have them scatter turn invisible and hide, then attack with ranged poison (which can kill if the target rolls bad on their save).
- 8 blink dogs, CR 1/4, can teleport a short ways and attack, but 8 wolves is still probably better although these have twice the HP
Teleport and attack is pretty darn powerful.
So, none of them are "terrible" or "useless" like you described them. Two of them are more situational than the others (sea hag and sprites). The rest vary from "Just fine" to "Damn good", like I said. The situational ones are excellent...in their correct situation (and scouting isn't rare and the sprites can still be effective in combat, nor is "death gaze, but need to prep for it" that weird either).