Implementation was rather bad on Linux despite reports and warnings of extreme breakage during the beta, and the legacy client was the only escape hatch for many after they bricked it following force pushing the beta into the mainline client, with no rollback option. If you follow Valve's bugzilla tracker, you will see hundreds of bug tickets over the last month relating to this UI, which didn't account for regressions around CEF, Gnome, upstream Nvidia drivers, focus-follows-mouse, memory leaks, screen readers for the blind, instability with system dependencies (Steam carries around a basket of old dependencies), and more. Here is but one example: https://github.com/ValveSoftware/steam-for-linux/issues/9805. Steam only targets Ubuntu and seemingly only tests on that platform, and in so doing this causes a myriad of problems. It's not an aesthetic issue.

Speaking of memory leaks, one of the most egregious issues right now is the client rapidly consuming all VRAM within a matter of minutes when the window is interacted with (https://github.com/ValveSoftware/steam-for-linux/issues/9638), effectively killing the system and making rendering games impossible.