Wizards can be quite powerful under the right circumstances. Can they go nova, burning off spell slots willy-nilly?
Yes, because 5e is not designed in a way that supports meaningful punishment for this, unless the DM becomes actively hostile and clearly playing fast and loose with diegesis (because three hour-long short rests have no consequences, but one eight-hour long rest is totally a bridge too far, every time, forever? And that's somehow not blatantly and aggressively gamist?)
Do enemies always approach in fireball formation?
The way 5e combats are designed? I mean kind of.
Fireball has a 20' radius. That's easily 40 five-foot squares. Not hard to catch the enemy in a 20' radius explosion.
Ignore that the fireball sets unattended object on fire?
Not sure how that's actually relevant. The vast majority of locations aren't meaningfully occupied with flammable objects to begin with.
Are their spells never countered?
Rarely, because
counterspell is both dull and tedious. There's a reason it's frequently complained about by 5e DMs.
Are the monsters resistant or immune to the spell damage type?
Then pick a different spell. Two or three solid damage spells is enough to avoid most relevant resistance. E.g.
fireball and, say,
ice storm.
Can the wizard target the enemy without affecting allies?
I find many Wizards do not consider this a meaningful disadvantage.
8d6 isn't really all that much when the fighter can do just as much damage to an individual every round all day.
What Fighter is this? Because 8d6 to a single target is only if you hit with all four attacks with a great sword at max level. Even if we factor in static damage (so 2d6+5), you're still talking about an 11th level Fighter, at which point the Wizard is dropping (up to) four
fireball*s a day, plus three 4th level spells, two 5th, and one 6th. Even if the Wizard is foolish enough to waste a *fireball on just two targets, and one of them saves (or three targets that all save), that's still 12d6 = 42 damage from an action they can perform four times a day (without considering Arcane Recovery.) 168 daily damage. At that level, a Fighter even with a 75% hit chance (70% hit, 5% crit) is doing .7(12)+.05(17) = 9.25 average damage per attack, or 27.75 damage per round. It takes her five rounds, counting Action Surge, just to make up for
one poorly-used
fireball. That's 20 rounds of combat per day,
just to keep up with 4th level spell slots used in fairly inefficient situations, despite assuming extremely high accuracy and a high rate of enemy saving throws (50%). Champion doesn't meaningfully affect this (hence why it's not a very good subclass), BM helps but not a ton, as it rarely gets more than about half the described
fireball (two targets, one saves, or three targets where all save), at 82.5 average extra damage per day.
And that's JUST using four (rather poor) *fireball*s and otherwise leaving all other resources untouched. This Wizard can bring two more *fireball*s, or an extra 5th and 1st spell, with just one short rest, and has several 4th level spells to bring to the party as well. And all their 1st and 2nd level slots can be used for... whatever. Without even considering rituals on top of that.
When paired with the fact that
most groups don't actually have 6+ combats per long rest, nor 2+ short rests per long rest, I think you can see the shape of the issue. And yes, this is a real thing. Crawford himself explicitly said so, when talking about how some classes, like Warlock, get left behind because they aren't given enough short rests to keep up with Wizards.
People wanted nearly the same rules as 3e, without LFQW and the 5MWD. And guess what we got! Wizards still grow faster than Fighters can keep up, and the 5MWD is still a pervasive problem.