I'm going to go through stat by stat, and point out everything wrong with this tank.

DPM

Arty is unique in that its tanks reach peak DPMs at mid tiers, and then tend to drop off. You can guess which arty has the highest DPM in the game.

leFH18B2: 2645
Crusader SP: 2250
Bishop: 2093
FV304: 2000
AMX 13 AM: 1907

17.5% higher than 2nd place, and 26.3% higher than second place in its tier. It isn't even remotely gradual.

Penetration

Arty HE shells peak at 60 mm of penetration.

T92 HMC (Tier 10) with 60 mm.
Conquerer GC (Tier 10) with 59 mm.
leFH18B2 (Tier 5) and a few Tier 8 and 9 tanks with 53 mm.

For reference, the HE penetration of other Tier 5 SPGs ranges from 28 mm to 38 mm.

Reload Speed

The leFH18B2 has a reload time of 8.92 seconds, which is enough to perma-track enemies. The only tanks with comparable reload times on their top guns are Tier 2-3 SPGs (plus the Birch Gun). Those tanks are balanced by having 19-21 penetration and 165-180 alpha (the Birch Gun having 22 and 280 instead, respectively), often dealing pitiful damage or none at all.

The leFH18B2 has a penetration of 53 and an alpha of 410.

Shell Types

The leFH18B2 is one of only two SPGs with access to AP and HEAT shells, the other one being the Sexton I. So on top of being a premium with some of the highest SPG HE pen in the game, it also has access to gold ammo (in a class where penetration is one of the primary disadvantages), for double the pay-to-win. In-game, this basically means it can penetrate anyone that it comes face-to-face with, and is one of two SPGs that can reliably penetrate the top of tanks like the KV-1.

View Range

This is probably the most egregious one, as the leFH18B2 has 390 view range, the most for any Tier 5 tank: the second highest being 370 m. It is in the top 10 of all view ranges up until Tier 8, where the rest of the tanks start to catch up.

Now, that was compared to other classes. For other SPGs, the leFH18B2 is the best of all SPGs in the game, by a ridiculous margin. The second highest SPG view range is the Sexton I (a premium), at 330 m. The third highest are some Tier 10 SPGs, at 310 m. It has at least 26% higher view range over any non-premium SPGs in the game. The view range of other TIer 5 SPGs ranges from 250 to 265 m, which equates to the leFH18B2 around a 50% view range advantage.

Survivability

The leFH18B2 is tied for the 7th highest frontal hull armor of all SPGs in the game, at 60 mm. But for sides and rear it's tied for 2nd place.

The leFH18B2 is the heaviest tank of the Tier 5 SPGs. In combination with its armor and above average HP, Tier 5 light tanks will kill themselves trying to ram it.

The leFH18B2 has 120 track health, 50% more than the 80 most of its Tier 5 bretheren have. This is on the same level as Tier 7 SPGs. Track health is important because it makes the leFH18B2 more resistant to tracking (from counter-artillery fire or approaching scouts).

Gun Traverse

Getting a bit nitpicky, but the leFH18B2 is tied for the highest gun traverse speed of all Tier V+ SPGs, at 22.94 degrees per second. For reference, the M44 has a gun traverse speed of 10.43 degrees per second. The leFH18B2 also has the second highest gun traverse arc for its tier, at 15 degrees in either direction.

Bonus Round: Bishop Comparison

The Bishop is the closest equivalent, sharing the same Tier and some of the advantages, like being the 2nd highest DPM in the game.

The leFH18B2 has the following advantages over the Bishop:

Almost double the penetration.
3.45s faster reload.
52% higher shell velocity.
Slightly higher top speed, turret traverse, and hp/t ratio.
Almost double the weight.
Better side hull armor (60 vs 50)
50% better track armor and track health.
56% better view range.
Half the shell cost.
Access to AP and HEAT shells.
All the premium benefits.
Much higher range (the Bishop has a max range of 500m, which is less than normal tanks due to being lower than the maximum render distance).

The Bishop has the following advantages over the leFH18B2: