is there some problem when using the 2d map built by slam in uneven ground?

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There are different solutions to work with uneven grounds while performing SLAM.

For example there is ROS package from ANYbotics for elevation mapping:
https://github.com/ANYbotics/elevatio...

The software is designed for (local) navigation tasks with robots which are equipped with a pose estimation (e.g. IMU & odometry) and a distance sensor (e.g. structured light (Kinect, RealSense), laser range sensor, stereo camera). The provided elevation map is limited around the robot and reflects the pose uncertainty that is aggregated through the motion of the robot (robot-centric mapping).

Another solution is to use 3D octomap to capture elevation information as well: https://octomap.github.io/

The OctoMap library implements a 3D occupancy grid mapping approach, providing data structures and mapping algorithms

You may also look at another ROS package Cartographer.

Cartographer is a system that provides real-time simultaneous localization and mapping (SLAM) in 2D and 3D across multiple platforms and sensor configurations.