I consulted widely before settling on a company with an office in my neighborhood to redo my roof. During the process, I heard of a group of Chinese guys who were the cheapest. But they spoke no English - the explanations were to be done through an interpreter. I struck them off my list right away. Another company couldn't guarantee my garden wasn't going to be damaged. Off the list.
In the end, I received a no small print 15 year warranty directly from the shingles manufacturer and 15 year labor warranty from the roofer, all of which are transferrable when I sell my house. Paid a little more for peace of mind.
Hopefully for you they honour their warranty.
Written or not doesn't mean shit if they choose not to warranty it for any number of reasons in the future. Ask me how I know.

