It can work. I work in the financial services sector. I instructed part time for 10 years working at some very good schools including intergrated, never moved the flying on to bigger things as I was a little old and was scared of spending more money on it.
2008 came and I found it difficult to get work. I built a fair bit of experience. I usually did Fridays and when required. Did complex conversions over the weekend to France etc it was incredible fun. Money was quite good then but I understand the rates have come down was getting 25 - 30 an hour PPL and 40 - 50 instrument complex then, but I know instructors who are instructing for free now.
I now have set up my own financial company but would love to get back to instructing as I miss it greatly, but it can be difficult to get regular work so good to have a another income.
I think instructing full time would be hard work as I did two weeks cover etc and was usually knackered. I know most see instructing as a means to an end, but it can be really fun and I would go back to it in an instant if the right opportunity came along.