Drawbacks are:
- Lack of job security
- Loneliness (always an outsider)
- More responsibilities (pension, taxes, vat returns, chasing invoices, legal etc.) It all gets pretty draining
- No sick pay
- Contract writing/negotiation
- You often end up working longer hours to get things done and keep people happy
- Getting a mortgage or loan in the UK is generally more difficult if self employed, even if you're making bank.
The other thing I'd add is that to be a good contractor/consultant you need to have a well rounded skillset that extends far beyond the technical. The ability to make any sort of impact at a larger business where management are too fond of meetings, marketing are completely change-averse and engineering just want to see you fail has very little to do with your ability to engineer software. You need to get decisions made, win confidence and support and integrate with teams very quickly - it can take you away from doing what you love sometimes.
Basically, I'm glad to have the money right now but I'm not sure i'll do this forever. The comfort/security of normal employment is sorely tempting.