What are the drawbacks of putting down pavers for an alley without a sub-surface?
No tips other than doing it properly.

Otherwise the pavers will be all wonky and wavy within 1-2 years in most cases.

You could try digging down the height of paver, compacting a little, putting pavers in and fill cracks with dirt and planting grass in the cracks. No guarantees from me on the result over time though. Chance of this working out would completely depend on your soil conditions and weather. I've done this with flagstone and it has worked well.

Also a geotextile fabric would help in keeping uniformity.