I have tested several HDDs and SSDs connected to a Wii and noticed the following details:
Here in Brazil, there are many cheap USB 2.0 HDD cases available on the market. People often buy them to use with their Wii but end up facing issues. HDDs and SSDs in these cases often fail to work properly. HDDs sometimes don't start and you can hear a clicking noise, with no apps appearing on the Homebrew Channel (HBC). In contrast, SSDs usually start correctly, and apps show up on HBC, but when trying to launch USB Loader GX, it often freezes on the message "Reinitializing Devices" (on WiiFlow, it freezes at the logo screen).
The easy fix for these issues is to switch to a higher-quality case, preferably a USB 3.0 one, assuming the HDD or SSD itself is in good condition. Alternatively, using a Y cable might resolve the issue.
It's rare for HDDs to have problems when used with a good-quality case (even better if it's a genuine external HDD). However, some SSDs may still show the "Reinitializing Devices" message in USB Loader GX. The only solution I've found for this is to format the SSD to WBFS.
Does this mean HDDs are more reliable?
When both HDD and SSD are working correctly, I haven't noticed significant differences between them. SSDs do start slightly faster (1 to 3 seconds quicker than HDDs), but I haven't observed any noticeable difference in game loading times.