When designing items for games, be it weapons, cards, or anything else, are developers given rules for hard limits of what that item can do? Say, a gun cannot have a DPS above X, a card with cost Y cannot have more than X/Y stats without a downside, a wizard staff cannot have the slashing attribute, etc? If so, how are these rules decided?
We don't really have hard limits per se, but we have guidelines we try to follow. What you're talking about is a power curve which, in game design, describes the way that game elements grow in power over the course of the game's advancement systems. It is rare for a game to have items that are always obtainable from start to finish; usually there is some sort of progression from early game to mid game to elder game with growth in power along the way. The amount of growth and amount of resulting power is usually some sort of math formula, with small independent tweaks here and there to increase interest and fun. Usually this is a formula that takes some sort of resource cost or requirement (e.g. level, mana cost, gold cost, drawback, etc.) and converts it into some sort of desired result (damage, stat boost, good effect, etc.).
System designers take the costs and the desired results and create a formula to calculate one from the other, based on how we want the items at that power level to play. As an example, in Hearthstone minions that cost one mana have a rough ceiling of some established power. Minions that cost two mana have a higher power ceiling, minions that cost three mana have even higher, and so on and so forth. This is our baseline power curve - the guidelines that we use to generate our first pass for stuff. We establish a band at each cost for acceptable power - a low end and a high end, so that we have a means of measuring items that are too strong for their cost and items that are too weak for their cost. The floor is typically pretty low, really weak items generally don't cause gameplay problems like overly powerful items can.
We then add in modifiers like rarity (e.g. a three-cost legendary minion is better than a three-cost common minion) to adjust the power level of the individual items. We can add in interesting procs, beneficial effects, drawbacks, or whatever at this point. Each of these benefits or drawbacks has a power budget associated with it we use to determine its position within the power band. We can adjust these budgets to make certain options better or worse at our discretion, with the goal of staying within the acceptable power level band.
We then playtest our work to see how our numbers and power levels actually feel. We take our observations, iterate and adjust the numbers, and then playtest again, repeating the cycle until we get our desired play experience at each power step and level.
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