It's only really overpowered when NPCs use it against PCs. Enemies normally die so quickly that it isn't broken and most of them don't wear armor.
As for it being worse than those other spells, that's kind of debatable. Conjure Animals isn't as good as everything thinks it is. There's basically one or two animal choices that make it good.
We are all very much aware of the official spell interpretation. Your assessment of its power is not even remotely correct; and more or less proof you haven't actually seen the spell used. Every attack used against the animals is an attack that is not leveled against the caster or their (original) allies. Most summoned beasts can survive multiple strikes from both PCs AND monsters*. AoE abilities can occasionally remove them all - except that the caster gets to choose where the animals appear, and whether they appear in an appropriate formation. The damage output of even the weakest CR 1/4 beast is considerable when there are 8 of them.
* (For at least the first few levels that Conjure Animals comes into play. Higher-CR monsters can eventually one-shot the summoned critters of course, but the spell can be upgraded to summon more of them so that the attacks neutralize a smaller percentage of the summoned forces).
Spike Growth, IME has no effect at all. You cast it, all the enemies stand still after the first one takes some damage from it, and they use range weapons on the caster until it goes away. Flaming Sphere only does damage at the end of a round that you are next to it, so anyone will move away from it, thereby taking no damage at all. Basically, it is a single target spell that requires a bonus action every round to attack someone.
My experience differs sharply. Many monsters have either no ranged attacks or ranged attacks that are very considerably weaker than their melee attacks. It is furthermore very easy to deny or impose penalties on ranged counterattacks by dropping prone, moving behind companions, or moving behind walls. On top of this, there are a plethora of attacks and abilities that force movement through the area. Including for example: Thorn Whip, Thunderwave, Tidal Wave, the Battlemaster's pushing attack, the paladin's Thunderous Smite, the open-hand-monk's Open Hand Technique. Not to mention the variety of other spells and effects that cause damage if enemies within the spike growth area do NOT move.
Flaming Sphere deals damage BOTH when it is rammed into an enemy (as a bonus action) AND when an enemy ends their turn adjacent to it. While it is frequently simple for enemies to end their turn away from the sphere - it is also occasionally possible to place the sphere in a position where enemies cannot BOTH move away AND avoid attacks of opportunity from melee combatants. In contrast to Heat Metal, Flaming Sphere does not require enemies to be bearing metal armor or armaments, can affect multiple creatures per round, and can be retargeted once the initial enemy has fallen.
Heat Metal still has its (niche) uses. A lizardfolk shaman casting the spell on the party fighter and then ducking down into the swamp muck while the unfortunate warrior cooks is certainly amusing. But it is not the game-breaker that some would claim it is.
