Here’s what the armory tells us about the effects of upvetting a unit:
-19% stun effect duration: An elite unit suffers far shorter stuns than a rookie one, but neither is more resistant to being stunned in the first place.
+5% chances to see and identify enemy units: This does not increase spotting ranges. As far as I know, what the bonus actually does is make it more likely for a spotted but unidentified enemy unit to be identified (changing label from “…” to “M1a2” for example).
+150% faster morale recovery: What it says on the tin, elite units go back to calm state faster than rookies.
-10% dispersion on artillery shots: What it says on the tin.
+8% Accuracy: This is the only important bonus of the lot, and to make up for it it is really important. There’s a couple things to know about the accuracy bonus:
- The armory fibs a bit about how big the bonus actually is – armory tells us that elites get 32% accuracy, but testing reveals the boost to be closer to 50%. For example, an elite Kurnass with 50% innate laser bomb accuracy has a 73% chance to hit a machbet (medium-size) from max range. I’m not aware of an exact formula for the actual amount of accuracy each veterancy level provides, but the heuristic I use is that it’s a bit higher for each level than the armory says it is and that there’s an especially big jump between veteran and elite. By the way, this fibbing should also make you suspicious about the other modifiers listed.
- The bonus is multiplicative with the base accuracy. A 20% accuracy vehicle with say 10% more accuracy from trained won’t have 20%+10% = 30% effective accuracy, it will have 20% * 110% = 22% effective accuracy.
- The bonus is additive, or something close to it, with other bonuses or penalties. This is important because it makes it effective at wiping out ECM or panic penalties. For example, consider shooting at a 50% ECM target with 50% elite accuracy bonus and 100% base accuracy. If the bonuses were multiplicative with each other, we’d get 50%*150%*100% = 75% effective accuracy. If they’re additive, we get (150% – 50%)*100% = 100% effective accuracy. Ingame behavior is closer to the latter case, making veterancy really helpful for AA pieces and ASF.
To finish the post, here’s a graph by nandemonai, on the effects of veterancy and ECM on accuracy. Each color has two lines, the top one represents elite veterancy, the bottom is for rookie: