When installing downspout drain lines in the soil, never bury them immediately adjacent to a home that was recently built. The soil around the foundation can take five to 10 years to self-compact. In loose soil, pipes can bend, kink, or develop reverse drainage. If you use the slip joints, the pipes can come apart at the joints.
In new construction, try to put as much of the pipe system as possible in undisturbed soil. If you must run a pipe through disturbed soil, give that section of pipe extra slope, up to an inch per foot of run, to compensate for future soil settling.