Wind energy is unreliable because: 

Wind power is a variable resource, and the amount of electricity generated at any given time is determined by wind speed, air density, and turbine characteristics (among other factors).
If the wind speed is too low (less than about $2.5 \mathrm{m}/\mathrm{s}$) the wind turbines cannot produce electricity, and if it is too high (more than about $25 \mathrm{m}/\mathrm{s}$ ) the turbines must be shut down to avoid damage.
As a result, because the wind does not always blow and its speed changes abruptly, wind energy is unreliable.