In these cases, the contractors will often remain on their jobs, just change employers. Working for a contracting agency has the advantage of flexibility and disadvantages of a less secure position and lower pay. Benefits may also not be as good. Pension plans take some time to vest. Working for a small company is probably the best way to gather a lot of experience quickly.
Small companies have fewer resources to respond to sudden increases in workload.
Some, especially startups, have extremely limited resources to meet even regular demands, so everyone has to work very hard. Stories of 70 and 80 hour weeks are told with some regularity. Job security might also be shaky. A coworker told me of a morning when he came to work to find the doors locked - the company had gone bankrupt overnight. On the brighter side, experience gathered in such an environment might take much longer to acquire elsewhere.