In the early game, no unit is more valuable than rocket artillery. It specializes in bombarding unarmored units, when that is what everyone has. For Pan Asian in the early game, it's not as valuable because of their bonuses for regular artillery.
As the game progresses, SP arty and SP rocket arty become more important. Comintern get doctrine bonuses on SP rocket arty and Allies get bonuses on SP Arty. SP rocket arty are powerful in the mid game, but are rarely worth upgrading due to how common unarmored units are in the late game. Whereas, SP arty become more powerful in the late game due to how abundant armored units become.
I've never used railroad guns for the simple reason that they don't upgrade. I see them as valuable in the mid game, but would lose their effectiveness over time.
Of course coastal bombardment with naval units is always effective, but will only get you so far.
Also, if you switch to the SP versions, it's rarely worth upgrading your regular arty or rocket arty except maybe as a late game luxury, especially when the research takes long and you don't think it would be worth researching and upgrading your premier units.
