I've got to hand it to you - just about every SINGLE claim made in that passage is nothing less than complete hogwash; aside from your personal experience which I obviously can't speak for. Having the DM choose which monsters appear does not suddenly make summoning less than overwhelmingly powerful.
No, but if the DM is disinclined to be co-operative they can have sharks appear in the desert.
Subjective: an uncooperative DM can make the spell completely useless.
I've personally played under well over a hundred, if not hundreds, of DMs since 5e came out.
You must change DMs very frequently then! 100 DMs in 6 years, I make that a new DM every 3 weeks. Are you waking away or getting thrown out? If you change games so frequently I don't see how you can get high enough level to cast Conjure Elemental (which, with it's 10 minute cast time, is useless in the average dungeon scrap).
Of those - the overwhelming majority don't bother focusing fire on a druid or summoner even with intelligent enemies.
Subjective: requires a generous or tactically naïve DM.
Of those that do, it's still rarely much of an issue because 1) not all foes are intelligent,
Situational: Some are, some aren't. Also depends on the DM since some favour intelligent enemies almost exclusively.
2) not all DMs handwave the fact that even intelligent foes may not be well versed in magic,
Unless it's a very low magic setting, even the dumbest troll can figure out that if someone waves their hands in the air and a pack of wolves appear then a spell has been cast.
Subjective: requires the DM to play monsters as exceptionally stupid and ignorant.
3) not all DMs handwave the fact that druids may not be easily distinguishable from the MANY other classes that tend not to wear metal armor (like rogues, rangers, warlocks, bards) or even those that tend not to wear any armor (like monks, wizards, sorcerers, and even some barbarians)
Even if the druid takes steps to disguise themselves (maybe by taking the flowers out of their hair), it's generally still obvious who just cast a spell.
4) many intelligent foes still do not have ranged attacks or attacks with sufficient range to hit those staying far back (or the ability to teleport),
If they didn't prepare for eventualities then they wouldn't be very intelligent. Even the dumbest goblin carries a shortbow.
5) many intelligent foes with ranged attacks do not function well when melee combatants or, say, a swarm of animals are directly in their faces,
True, but saving throw spells work just fine. And a pack of goblins could hold off a pack of wolves plenty long enough for the archers to shoot the druid. Or the dragon could breath weapon all your wolves without even bothering with the druid.
Situational: depends on what you are fighting
6) intelligent foes that focus fire on summoners often telegraph their intent by talking amongst themselves in languages the PCs can understand, allowing the caster to take appropriate countermeasures.
Which is the problem. In 5e the (non-moon) druid has very few possible countermeasures, with Barkskin and Stoneskin having been nerfed into oblivion. Wizards, Sorcerers and warlocks get countermeasures aplenty.