Granted, not everyone might see them as good, and a lot of people's opinion probably comes from other people talking about them rather than experimenting with them in a real game.
Without going into details, and save for the few early levels, during which you might have seen a few skunks being conjured to great effect, a top-level summoning slot brings up a creature between 4 and 5 levels below the party.
Due to how encounter math works, a creature of this level is counted as between 0 and 10 XP in the rules for building an encounter as its chances to hit are too low to matter against the player (-5 to hit against +5 to all defense at a minimum, often more from proficiency upgrades).
Of course, that's for abilities targeting defenses, surely I just have to pick things that don't target defenses or satisfy myself with spawning an annoying flanking/body blocking buddy? This is correct, some very select support-oriented monsters, like the Satyr or, in an undead campaign, the Deathless Acolyte can give an amazing boost for their level in a vacuum; but that's before considering what truly seals this pan of the game for me
It's woefully action intensive for the caster. A good way to see it is to say that you're spending 3 actions to slow 1 yourself in order to add a level -5, stunned 1 monster on your side of the board, and if the support action of a Satyr might feel pretty good, is it really compared to other uses of 1 action for the caster, like using a composition cantrip, an appropriate metamagic, or using a well-chosen skill action like bon mot or demoralize? and that's excluding the initial 3 action opportunity cost you could have spent on a more potent spell
In short, there is a reason why level -5 creatures don't count in the encounter budget, and while a well-chosen one might impact the fight positively, 2 of its actions are almost never going to be better than 1 action of a creature 5 levels higher;
Of course, that doesn't mean the spell is useless, out of combat in the blood lord adventure, for example, a single cast at 4th level of animate dead can be used by the Wizard to heal everyone for 20 + 3 x (2d8+16) to distribute on the most injured in a minute with a deathless acolyte; that's amazing, and notably way more than the 0 a wizard would be able to provide otherwise. Similarly, if you know something is booby-trapped and you don't want to risk your rogue, a Crawling Hand will happily eat and "disarm" it for the party for the cheap price of a 1st level spell.
Summoning was specifically defanged in combat, probably as a design concern about minion spam that was prevalent in previous editions, so just... don't use it in combat and demoralize/bon mot every turn instead, you'll be doing more good for your party