I will be a nephrology fellow in ~gash~ 7 months and have talked to many people in the field from all spectrum of practice. And my feeling towards the topic about income/lifestyle is always as follows: any field in medicine is what you make of it. You can bust your butt in pp and make a $hitton but have no life, or take it relatively easy and make "average". It's the truth of almost any field, even if you go to primary care pp, you can expect to bust your chops 60-80/wk if you want to make a good amount. In the age of shrinking pie, every field (especially the fields that are currently lucrative) is going to eat a cut of some shape or form. Ultimately you gotta do what you enjoy doing. Overall nephrology is not a life-style oriented subspeciality - not the harshest but by no means cush at all. And as a fellow (especially first years), you do bust your chops almost regardless of where you train.
With that being said, I think in terms of longterm outlook, depending on area of the country, the jobs are more or less there. Just like every field, you will find more saturation in metro areas and more vacancies in the rural areas. Heck, even right now before I am even started, I am starting to have recruiters email/calling me promising me 20k signing bonus or loan repay if I sign on to join nephrology groups in rural midwest towns, many have pretty decent incentives too. Given the overall aging population, job security is very much there, unless they start allowing NP's to dialyse people which I dont think will ever happen. (At least so far) I do not regret commiting a career to nephrology in any way, and am overall optimistic about its general outlook.