Stress
Any job, even a job you love so much you'd consider doing it without pay, will be stressful at times. Some low-level CSIs have to observe how the animals are put to death. Gruesome? Yes. But the CSI is in charge of making sure humane methods are used.
CSIs may work in hot conditions (inside or outside), but most work in fairly cold conditions. If you are so cold-sensitive you didn't even want to go see Frozen, then CSI work is probably not for you. Keep in mind, though, CSIs wear protective garments over their clothes, shoes, and hair, so it's not as cold as you might imagine.
As a CSI you may have to work on your feet all day, travel a lot, deal with shift work, and even work holidays like nurses and police officers (source).
And you don't even get to carry a gun.