If it is like the Wii U older titles will also work. Games like splatoon, mario u (we love playing this as a family)and all older titles (via emulation) should be available.
Why would you assume that wii u titles would work? The switch only takes cartridges, virtual console titles are already tied to the console and don't even carry between wiiu systems, and the switch uses an nvidia tegra (arm) architecture where as the wiiu uses powerpc and an AMD graphics chip.
Not even the wiiu controllers will work with the switch.
Only if they are available digitally (no DVD slot for Wii U discs) and only if Nintendo offers some digital game transfer program like they did between the Wii and the Wii U (digital games are tied to your console, not an account)
Not even the wiiu controllers will work with the switch.