A significant portion of the games involving upgrade systems are guilty of at least one of these three sins...
Unknown Improvements
Vagueness
Some games have upgrades. Some of these upgrades, for example, result in increases to damage - but sometimes the descriptions are vague and don't give much detail on how much more damage you inflict. Not something encountered that often, but when it does pop up it's rather jarring.
Pretty and Meaningless Bars
A lot of games that have upgrades (and several games that don't) have stats represented by bars. As a parameter is higher or lower, that stat bar increases. However, in a number of games there's no measure of how much more of an improvement a full bar has over an empty bar.
Take Alien Swarm, for instance. Its characters all have bars and even fractions measuring how effective their respective skills are. However, a certain comparable skill may only be shared in between two characters, making the bar useless for comparison - "More effective Motion Spotter" would convey a similar amount of information.
In general, however, sometimes bars are just that, bars - with no way to measure exactly how much a parameter affects combat. That pistol you just unlocked has a full centimeter on your old weapon as far as power - but how much more damage will it inflict? Can it kill on a single headshot?
Confusing Percentages
Mad Max will be my example here. Max's elbow guards and knuckle upgrades both allow Max to parry more effectively (and receive less damage from failed parries) and inflict extra melee damage, respectively.
However, the upgrade descriptions for both items describe this increase as a percent increase/decrease.
You do not know how much damage your attacks deal in numbers. In other words, all you know about these upgrades is that they make enemies die faster (or make you die slower).
Same happens all the time when percentages come up - and you're not given any other numbers.