Given a) Switch is more powerful than Wii U and b) the Switch version is running off a cartridge versus the Wii U version running off a disc, I would think a worse frame-rate on Wii U is to be expected. What open-world game doesn't launch with frame-rate dips, and in many cases, severe bugs?
The only one that comes to mind for me is Xenoblade Chronicles X. With the data packs installed, I didn't have frame rate dips, though pop-in and loading times were still an issue. I'd be interested to know how Gamekult are playing the Wii U version. Are they using a download code from Nintendo? Are they using a disc with the 3GB install, or a disc version without the install? Now that I think about it, does the 3GB data install from the disc, or is it downloaded from the eShop ala Xenoblade X? All of these factors will alter the performance of the final Wii U version. If you want the best performance, you'd be better off downloading and running Zelda from Wii U's internal memory.
If the frame-rate issues in Wii U are so bad as to be unplayable, then I'd be concerned and deeply annoyed. While I do expect the Switch version will be the superior version, I also expect the Wii U version will be fine.