It is done when there is no alternative.
Be aware of four things:
1. When the power is off, you have no pumping. The amount of water that you can use depends on the capacity of the sump. If you are on a well, you may not have any water anyway. If you use a generator to supply water, plan on connecting the sewage pump to it. With a gravity system you can flush a toilet with a bucket of water from the stream. You can't do that if the electric sewage pump isn't working.
2. You will know quickly when the septic system fails or gets flooded because of ground saturation, because water and sewage flow downhill.
3. You will have to deal with a replacement pump from time to time. You should learn how it works, what it costs to replace it, and the make and model number of the pump. Know who is going to service it. Make sure you are getting a very good and reliable pump with capacity margin; not a piece of junk that will just make it past the house warranty.
4. A pumped septic system, rather than a gravity system, should REDUCE the value of the home by $2000 to $3000, which is the present value of your future costs for electricity, reduced reliability, service calls, replacement pumps, and reduced resale value. Part of that could cover the cost of an emergency generator.