Self-employed people earn a higher income on average than people who receive a regular paycheck, but they can have a harder time getting approved for a mortgage. They can also find it more difficult to get approved for a loan modification if they fall behind on their mortgage.
Some lawyers might tell you that you can't get a loan modification if you're self-employed. That's just not true. You can. However, it will most likely add more work to a process that already takes a lot of work.
The reason getting and modifying a mortgage is more difficult for self-employed people is that their income varies more than salaried and hourly workers and they don't have the pay stubs or W-2s that make it easy for banks to verify earnings.