I would have to disagree, IMHO a greenhouse is a wonderful thing to grow in, if you have the right set up. Humidity can be controlled the same way as a normal grow room, with air intakes and vents. Temps can be controlled in the same way.
I like to consider growing in a greenhouse more like growing inside then growing outside, because plants grown in a greenhouse take a lot of tending to. Like Hick said the purpose of a greenhouse is to control the environment, just like an indoor grow, but to use the power of the big halide in the sky.
If you are looking to plant and forgot about your plants then planting in a greenhouse is not the best way to go, but if you have time to devote to your plants then i love my greenhouse.
You will need to get a shade cloth for those super hot days, and have your greenhouse wired to your house, or a solar generator because you need electricity in your greenhouse for fans and ventilation.
I will say this though, if you are trying to grow 100+ plants in a greenhouse, that is not a good idea. I don't know if you have ever seen cannabis grown in a greenhouse, but it can get very, very, very big. I mean like 7 feet high, with atleast a 1 meter square diamater. With a good yeilding strain, you could possible get 1 pound to 1 kilo per plant. To grow 100+ plants in a green house you are going to need a greenhouse that has atleast 350sq meters of growing area, that is huge and would standout like a sore thumb.