Though most of it is already mentioned, I want to write it out anyways on my take.
Starting with UC:
These Zeon suits in my opinion were more realistic. The design wasn't too fancy and weapons equipped made sense (i.e. the 120mm machine gun which is most commonly used for standard Zeon suits, panzerfausts, standard bazooka, basically projectiles). Also, not all Zeon standard suits were fitted for every environment scenario. Some had to be remodified for either ground or space use. For example, in MS Igloo: The Hidden One Year War ep 3, after Operation Odessa, the Type J Zakus were helpless against the Federation's Ball units because the Type J was built specifically for ground use.
CE:
Every suit paying homage to the UC were able to operate in both space and under gravity. Even the first generation ZAFT suits the GINNs were able to perform the same under gravity as they do in space sans the ability to fly on Earth. Also, CE Zakus, Goufs, Doms were all beam weapon crazy. Although these are because of technological advances, it just seems unrealistic how in only 2 years they were able to mass produce beam weapons to grunt suits while the UC era took at least 7 years even with the captured Zeon technology. The designs copying the original Zeon suits aren't too bad but once I saw the Zaku Warrior, I thought, what? Are the writers running out of ideas? I mean, even though ZAFT suits have the Zeon signature mono eye camera, I thought that the ZAFT suits in SEED were more original than the ones in Destiny.
I have to be honest here, I saw the CE era first and I thought the grunt suits were alright. After I saw the various UC era series, the Zakus, Goufs, and Doms just look totally more menacing than their CE counterparts. Example, the Gouf Custom vs the Gouf Ignited
In 08th MS Team, the Gouf Custom proved to be very intimidating, it held off 2 Gundam Gound Types, Ez8, and destroyed 3 Guntanks all by itself. The Gouf Ignited, it destroyed some other cannon fodder grunts, whatever, nothing special.