Slitherine

In Panzer Corps there was a pretty fair trade off between the lighter 10.5 artillery and the more powerful guns in that each artillery above the 10.5 had less ammo and a smaller rate of fire. In effect, 10.5 artillery did more suppression and had more ammo, whereas the more powerful artillery was more lethal but required you to restock the ammo more often. In Order of Battle the Long Tom does cost more command points but are there any downsides other than that? Are there any hidden rate of fire type mechanics or anything like that?

Costs 1 more command point, that's it. It's also quite expensive to upgrade all the 10.5mm Artillery to Long Toms, probably can't do that before Okinawa and IIRC the next scenario is also the last.

Depends how many Command points you have, 3 Long Toms vs 3 10.5mm is already the command point value of an Infantry squad.

Seems nobody else here has proper artillery!