Usually, a reputable third-party set like Kenko is a good alternative. Extension tubes don’t contain optics, so image quality isn’t affected by glass elements. The main thing to check is that the tubes include the electronic contacts needed for autofocus and aperture control, which Kenko tubes generally do.

Possible downsides are mostly about build quality and compatibility. Cheaper tubes may use thinner or less rigid materials, which could allow slight flex with heavier lenses and introduce alignment issues. Also, some older Kenko versions were not compatible with EF-S lenses, while newer ones are, so version matters if you use EF-S.

In short: for a known brand such as Kenko, there’s generally no major disadvantage versus Canon beyond possible differences in construction quality and lens compatibility.