My friend is starting to get some DT and is ordering the SSU since I have most of Axis and Allied. In looking at the unit cards online, I see for the SSU walkers their howitzers and mortars are classified as grenade weapons, yet Mickey's Howitzer and Steel Rain's mortar are not and the equivalent German weapons are not.

Should there be a change to the old cards/rules to level the playing field or is there a gameplay reason why SSU units will ignore cover while the previous units with similar weapons do not have a benefit?

I do not think so. Mickey or Steel Rain ignoring cover would be beasts. For gemany only unit i can think of is Hans and he does ignore cover.

SSU needs some packing in anti-infantry warfare couse their anti-tank capabilities are a bit broken on the ground until some of the hvy stuff hits the shelves.

 Zach's answer:

"The Grand'ma's Howitzer is a VK916 Howitzer, which is a superior form of the standard version carried by the Mickey. As a result, the Mickey's Howitzer does not gain "Grenade Weapon" status."

 …or historically reasoning, the gun Mickey/steel Rain has is not an indirect weapon and does not have the elevation to lob shells over cover.  Their method was to demolish the cover (building) that was being occupied, direct shooting at sort ranges.

 I can accept that. I will need to wait at least a month before his SSU units arrive, then I can see how bad it is for me. It just seemed weird for me when reading the cards.

Lska said:

How does the Hans ignore cover? is the Granate Werfer a UGL attack?

It's a "grenade weapon" like the UGL. The card is missing this info, but that was the official answer (and I think it's in the FAQ now?)