Homeowners can come across different issues depending on the type of tankless water heater they have - for gas ones, the burner may get clogged and cause hot spots to burn the heat exchanger. For electric ones, the heating element may need to be replaced if it is acting up.
This was the second plumbing company we had to try to determine where a leak in our house was coming from. They were very friendly and knowledgeable. Once it was found, they were upfront with cost and fixed it...