Well, I make a living out of being a sub. I sub in orchestras and other ensembles and in between I sub as a teacher, mostly teaching music but I've had everything from kindergarten crafts to grade 12 girl's gym (but that's a completely different thread in another forum!).
There are pros and cons to any profession and being a sub has them as well.
Cons: 1) you are only as good as your last performance. If you are going through a difficulty in your life and find it hard to concentrate and slip on a note, you are known forever as someone who can never concentrate and always slip up.
2) You are often on the road. This create wear-and-tear on the car and your relationships. Sure there are tax advantages but a tax deduction is still an expense you have to pay at the time.
3) Probably the hardest part is creating an income stream and a life just so that you can be available to sub. It's pretty rare to be called for 8 services per week, every week to create a good steady income to support a family and make the bank happy. This might be possible if you are single in a large city but in a small city like mine, it is unlikely.
If you have a full-time 9-5, employers don't look too kindly to you saying, "Um, can I have Thursday and Friday off from mopping the floors (teaching kids, selling clothes, processing credit card reports, etc.) so I can be on stage with Christopher Plummer? He'll be reciting Shakespeare while I play music by Mendelssohn and Prokofiev!" (While thinking, "It's really important to me and I'll make $50/hour doing something I love instead of $10/hour at something I hate.") They'll at you like you are a freak and say, "No, stop living in a fantasy and get back to work."
This can be very challenging financially. It creates a roller coaster of money that makes life difficult to manage and plan for.
So, my niche of supporting income is supply teaching and teaching a couple of guitar students at home. I also played in a couple of slice-above-community orchestras where I was the big fish. The schedules were getting in the way of pro orchestra gigs so I'm dropping them as well.
A friend of mine in Montreal is a landlord and plays with the MSO and other Montreal ensembles. Other friends teach their own or other instruments from home or at a music establishment (this creates scheduling problems as well).
4) Music work is often on the weekend and/or at night so it cuts into family time.
Pros:
1) You get to play with various ensembles with various conductors in different halls and venues.
Since there are so few of us, often the bassist will be the only one who gets paid!
2) You get to play interesting music that you may not get to play if you are tenured in a particular genre ensemble. If you are the principal bassist of the Johan Strauss Jr. Waltz orchestra in period dress performing 8 shows a week in Vienna, you are pretty much stuck doing that. Several musicians I know who played in Phantom of the Opera orchestra almost went crazy from doing the show over 6000 times! (Although I know of one who paid cash for a house in Toronto from doing that.)
You might even be the only bassist available who gets called 20 minutes before the show and you end up sight reading in front of the audience. You get looked at as the hero musician who "saved the show!" (true story)
Since the bass can play just about any genre of music (as compared to, say, oboe or viola), you can be called to do just about anything. I've played Polish polka shows, Middle Eastern gigs, Celtic kitchen parties, Jazz in clubs, the 12 Haydn symphonies with period instruments, standard Classical concerts, Film music, bombastic Romantic symphonies, Broadway type musicals, Opera, various Church gigs, Last night at the Proms fundraisers, backed up Rock bands, Chamber music, Baroque ensembles, Educational shows and so on. (On the down side you need to know how to play in the style of each of these genre as well. Fortunately, we can get away a lot of the times by playing the root and fifth on beats 1 and 3!

)
3) You can often have time off to yourself when others are grinding away at a 9-5 job.
4) There are tax advantages to running your own business.
5) If the people you work with at a particular gig are jerks, you are not stuck with them.
I'm sure there are others but I've written enough for now.