We also played our first games with artillery support for Motorized Infantry, but also realized that once motorized, they become vehicle class. Heck, we even gave the artillery support to Mechanized Infantry…
However, as you noticed, the +1 bonus if for infantry class units, not all infantry named units.
Since motorized infantry can move 2 and tow the artillery (or AA) and retain the infantry 4 defense roll, it does become a bit OP to have two attacks at 3 (Artillery + Supported), plus 2 movement, plus one artillery first strike for 8 IPP.
By comparison, the self-propelled artillery costs 5 IPP, so you would have to pay 10 IPP for the equivalent range and firepower, althought with an extra first strike and blitz capability, but one less defensive point (as both would defend at 3).
The 25% additional cost would not make S-P Artillery worthwhile, unless you have money to burn.
By losing the artillery support when purchasing or upgrading from regular Infantry to Motorized, you have at least have to choose between overall mobility and attack power.
Motorized purchases and upgrades are mostly for Germany and USSR in our games. The upgrade is meaningless in moutain terrain.
If you play the Spanish Civil War Expansion, the rolled event 3 “Failed Embargo” converts 1 Republican infantry unit to a motorized infantry unit. Considering that Spain is almost all mountains, this is more of a punishment, as you gain almost nothing useful and lose the artillery support.
