If your fighter can get there and attack, so can it. It's got a 30' movement.poppycock.
You cannot roll it into a group too easily.
It only affects the first NPC that it actually rams into.
Lets read that spell -
Any creature that ends its turn within 5
feet of the sphere must make a Dexterity saving throw.
The creature takes 2d6 fire damage on a failed save, or
half as much damage on a successful one.
feet of the sphere must make a Dexterity saving throw.
The creature takes 2d6 fire damage on a failed save, or
half as much damage on a successful one.
Now, while I get why you are saying "ram", you now have a device that is going to force them to move. Monster engages your frontline, you ram it and do small damage, but now he has to move or suck it again. Moving is a bad option because he has to suck an OA, or disengage. No 5's steps for free here.
Sure, one NPC often has 35% to 50% chance to save for 3 damage instead of 7, but it's a one encounter spell. It only damages creatures (including PCs) that end their turn next to it (not one that starts its turn next to it). Unless a foe has multiple PCs around him, most foes can easily get away. It's situational.
Might be for your group. My group personally has loved it in every edition and knows to make people stand next to the burning death. It is a strong spell. You do have to know how to use it.
It's also fire damage (which is the damage most likely to be resisted or immune to).
while it is fairly common, it is not that common. Don't bring a fire spell to a demon fight. That's why the rest of the spell book exists

Unlike Hunter's Mark, all of the NPCs know who is using this spell and they can focus attacks on the wizard.
Why? available to exact same number of classes. If he's being targeted, he probably already looks like a wizard

In a 3 encounter fight cast in the first round, and a 35% chance to save, it does 17 more damage. With a 50% chance to save for high Dex foes, that's 15 points.
Could you break out your math on this?
Hunter's Mark at level one can easily do 7 to 10 extra hit points per encounter.
Sure, and the ranger gets very few of those.
NPCs do not necessarily know that it is even happening. If the ranger never gets hit or never fails a con check, he can easily use this for 2 to 4 encounters. Sure, it stops with a short rest (shy of being used in a third level slot). But it's easily a match for Flaming Sphere.
I disagree. you can run it into the guy for the same or better damage, and he has to move, and it can affect multiple guys if they don't move. It's not the same role to be sure, but, as I said, the wizard's job is not to murder everyone. His job is to set them up. If you want to murder folks with spells, warlock or sorcerer is a better option.