The thing about artillery is, in our setting, FRAMEs are supposed to be mobile, fast and capable of rapid, hit-and-run strikes. Slapping a big howitzer on the back and sitting behind the lines of battle sort of runs counter to the design philosophy, and the role can be performed just as adequately, if not better, with conventional artillery. 

However, a FRAME with an auto grenade launcher or mortar can be useful, when you've got to saturate an area with fire on the way in. But it will be able to perform multiple battlefield roles, and not suffer one-trick pony syndrome. 

Large direct-fire pieces might come in the form of 60mm AT recoil-less rifles and bazookas; good for laying siege to sturdy positions and packing a wallop at close range.