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
Honestly, given how they seem to drop off at higher levels I've been considering homebrewing different rules for summons.
My current idea is to keep the scaling consistent, so something like (spell level * 2) - 3. Level 1/2 would be the same (-1, 1 receptively), but then it'd keep incrementing by 2 every level up till a max of level 17 for a level 10 spell. Another option is (spell level * 2) - 2, but I think that might be too strong at the level you get the creature.
I'll have to actually play with this rule at a higher level to see if it unbalances it, but IMO it looks like summons just get worse and worse at high level.
Something ive noticed at high leveld is that although the level gap gets wider, overall toughness and utility does actually keep rising. You're going to be hard pressed to hit anything, but ive anecdotally been able to summon monsters with key offenses that target key weaknesses to much greater effect at high level than at low level, and they often stuck around for much longer.