Keeping Shapeshifters From Getting Overpowered
The following is a list of checks and balances that I use to keep shapeshifters from getting overpowered. I don't necessarily use all of them on the same character, but using at least two tends to keep a character from going overboard.
Shifting is tiring.
Sometimes, the character can only go through so many shifts in a day before xe is too tired to shift any more. This means that the character has to budget xir shifting and put some thought into what to shift into, or whether it's really worth it to shift at all.
It takes time to learn new forms.
Before a character can zip into the form of a bumblebee on the fly, xe must spend considerable time and effort getting the form down. This puts a limit to the number of forms a character has, as it's not very practical to spend every waking hour of the day learning to shift into every conceivable animal.
Shifting is time-consuming.
Sometimes it's not instantaneous. Sometimes it takes minutes or even hours to complete a transformation, which means that changing form while in the middle of a chase or a fight is not an option.
Just because the character can take the form doesn't mean xe can use it effectively.
Each and every species is sized and shaped differently, which means that taking on a new form is akin to driving a new vehicle for the first time. And I don't just mean the differences between a sedan and an SUV - I mean the differences between a sedan, a bicycle, a helicopter, and a tank. A character who is perfectly practiced at being a wolf might change to the form of a coyote and trip over xir own four feet at first because coyotes are built differently than wolves. This also limits the amount of forms a character will likely use, as it's better to be effective at using a few forms than it is to be mediocre at many.
The character can only shift to a limited degree.
For example, a humanoid character can only change into other humanoid forms, or cannot change into exoskeletal creatures.
Circumstances beyond your character's control can force xir to turn back.
Your character may only be able to stay in form for a certain amount of time before being forced to turn back, or exposure to certain substances, temperatures, sounds, etc. may force your character to revert. This could potentially lead to very dangerous situations for your character - For example, transforming from fly to human while stuck in a sufficiently thick glass jar could be deadly.