You can buy SD to SATA converters that will take up to 10 micro SD cards and turn them into a "hard drive" (2.5" form factor, SATA connector).

In principle you could put 4x128GB SD cards in and get 512GB for about £100 using Samsung or Sandisk cards (Or 1TB for £200, 1.5TB for £300.) Cost wise that makes them a good percentage cheaper than equivalent capacity SSD drives.

How would this compare aside from cost? Is it performance that would be a win for the SSD, or reliability (or both)?

(I'm looking at this for a Ubuntu desktop but I don't think that justifies claiming it is on topic.)

In part I'm looking at it as a way to re-use SD cards that I have floating around but I'm not sure if it's worth the effort. At least for the ones I have looked at the total capacity will be (smallest card) * (number of cards) so if you have a mix of card sizes it doesn't work very well.