Ukraine

but how useful is it on a modern battlefield?

In general artillery is the most decisive military aid Ukraine could be given other than intelligence sharing of high value targets for drone/HIMARS/artillery attack.

The M110A2 is essentially a naval gun strapped to a small tracked caterpillar tractor, at 203mm it dwarfs the current large NATO artillery caliber standard of 155mm and compares to systems such as the Koksan (though it has far less range) edit no sorry the Pion is a far better comparison... except the Pion and Koksan are based on actual tank hulls so they inherently have better armor and chemical/nuclear warfare protection.

Truthfully I might classify this as less a howitizer and more of a "mortar" or combat engineer type weapon given it actually doesn't have that high of a range given its huge caliber size.

The M110A2 was powered by a Detroit Diesel 8V 71T, eight-cylinder, two-stroke, turbocharged 405hp diesel engine, producing a top road speed of 30mph (54.7km). Its fully ladened weight was 28 tons. The engine was at the front of the vehicle to the right of the driver’s position. The vehicle had a semi-automatic Allison transmission, which enabled the driver to pre-select four forward and two reverse gears. Access to the engine for maintenance was relatively easy compared with other armoured vehicles. Once the bolts had been undone on the two large engine covers, they could be lifted off by two men. Both covers had handles on them to make this process easier. The top cover allowed access to the engine block. The lower cover allowed access to the transmission, oil filters and final drive. What looks like two stowage bins built into the hull above the tracks on the left side of the vehicle are, in fact, access hatches to the air filters.

The running gear is slightly different to how other tracked vehicles’ suspension systems work. It does not have an idler wheel or return track guide rollers; rather, the track just sits on top of the road wheels. The sprocket drive wheel is at the front, while the vehicle uses torsion bar suspension. When the vehicle is parked at the designated firing location, the driver operates a hydraulic lock-out switch, which locks the suspension in place. This stops the vehicle from violently rocking when the gun is fired.

https://www.keymilitary.com/article/heavy-hitter-0

https://en.m.wikipedia.org/wiki/M110_howitzer

https://en.m.wikipedia.org/wiki/M115_howitzer

https://en.m.wikipedia.org/wiki/203_mm_howitzer_M1931_(B-4)

this soviet/Russian equivalent howitzer the "B-4" has a lot more fleshed out operational history described on its wikipedia, note that the firepower capability of the B-4 and the M110A2 are similar

The B-4 participated in the Winter War, with 142 howitzers placed along the front on March 1, 1940, of which four were knocked out. The B-4 was also called the "Karelia Sculptor" as Finnish pillboxes hit were virtually turned into a hodge-podge of concrete chunks and iron armatures.

23 B-4's were captured by the German 11th Panzer Division as the town of Dubno was captured at nighttime on June 25, 1941.

A total of 75 B-4 howitzers were lost from June 22 to December 1, 1941, and a further 105 howitzers were built from factories to make up for the loss. After the start of the Great Patriotic War, the Howitzer Regiments were evacuated to the far rear for protection, only returning on November 19, 1942 when the strategic initiative was wrestled back into Soviet hands. These howitzers would still be placed in the strategic reserve all the way up to the end of the war.

Captured B-4's used by the Wehrmacht were given the designation 20.3 cm Haubitze 503/5(R),[5] of which 8 pieces remained at the Eastern Front by March 1944, firing a mix of G-620 concrete-buster and German shells.

https://www.royalartillerymuseum.com/our-collection/self-propelled-howitzer-m110

https://man.fas.org/dod-101/sys/land/m110a2.htm

https://en.m.wikipedia.org/wiki/2S7_Pion

https://en.m.wikipedia.org/wiki/M-1978_Koksan

What makes these artillery systems obsolete isn't that they aren't effective as weapons, it is two things first that NATO and most professional militaries have trended towards around the 155mm artillery shell size as that is about as big as an artillery shell as humans can reasonably carry around. Any bigger than that and the entire logistics process including up to loading the shell into the cannon requires a whole lot of people or heavy equipment to assist which complicates, slows down and ultimately negates most of the advantages that come from having a larger artillery shell. This also means the fire rate of these very large caliber artillery systems only ever reaches about 1 shell per minute or so because of the difficulty of loading such a big shell into a cannon.

The second thing that makes this kind of artillery system severely obsolete for a modernized military is that there is ZERO protection for the crew either from small arms fire or FPV drones. Obviously in the event of a drone or artillery attack, if the artillery piece is not already mobile the best bet for the crew is to get as far away from the artillery piece as possible while under attack, but in situations where there is no alternative cover, no time, or the vehicle is on the move the crew protection is not only vital to keep the crew alive, it is vital for them to feel any degree of confidence operating in such a high threat environment with a veryyyyyy loud thing that attracts a lot of attention. While a truck mounted SPG like the Bohdana or Caesar also requires the crew to be outside the vehicle and exposed to fire, they have easily accessible armored cabs that can be reached quickly and they are also highly agile trucks that are much much much much much much harder to predict the movement of then a large heavy slow tank (unless of course we are talking about terrain that requires tracked vehicles). In modern warfare this kind of artillery platform just makes less sense than a towed artillery piece that can be entrenched, a self propelled truck style shoot and scoot artillery piece or a tracked, medium armor enclosed self propelled artillery piece like the Paladin.

For Ukraine though, this kind of weapon system is extremely useful and will most certainly save lives as even if this kind of artillery piece isn't used at the most intense frontline fighting one of the most vital uses of an extremely large caliber artillery piece like this is for bunker busting, it is for pummelling heavily entrenched positions that have already been isolated from enemy resupply to help disrupt the remaining resistance so that when infantry shows up to finish clearing out the defensive positions (unfortunately a necessary and costly operation, it is probably the most dangerous thing infantry can do other than clear out a large complex building) they aren't met with brutal, grinding resistance, they are met with people who just had a 203mm artillery shell dropped on their fortifications...

To the lives of the Ukrainian infantry being tasked with this kind thing you better believe it matters, statistically this is one of the best ways to systematically reduce the attrition of your forces, by using copious amounts of artillery to make sure that the enemy is shattered where you want them to be shattered so that your infantry don't have to dismantle the entire enemy army by hand themselves and pay that extreme price in lives.

As for the Zuni rocket artillery? The answer when it comes to the question over how much is enough when it comes to application of concentrated rocket artillery fire is yes, well just a bit more maybe.

https://en.m.wikipedia.org/wiki/Zuni_(rocket)