You press shift, and you get a birds-eye-view of the battle. Your reticle varies from circular to oval depending on distance to target, and several guns have maximum ranges that don't make it across maps (from what I've heard, that virtually disappears once you get to Tier 4 or 5).
Remember that your reload time can probably be measured in minutes with a ****ty crew, you have less armor than a tissue box, and your maneuverability is **** and you'll be more likely to survive.
SPGs are the ultimate glass cannon. High tier SPGs can one shot Tier 10 heavy tanks. I've seen it happen. The blast radii are huge, the damage is immense, and the range is extreme. Plus, the ability to fire over obstacles is not to be underestimated.
In short: They're difficult but devastating. And also very expensive to grind.