Old repairs can become the most visible part of the ceiling
Texture is forgiving; flat paint on a smooth ceiling is not. Patches that blended into a popcorn surface can show through once the field is leveled. Have the contractor identify areas of concern before work and explain how newly discovered defects will be priced.
If part of the ceiling is plaster and part drywall, confirm the crew is comfortable making the transition look consistent.
Build the finish sequence into the contract
The scope should explain protection, removal, patching, skim coating where needed, sanding, primer and finish paint. Ask whether cracks are being cosmetically filled or repaired with reinforcement, since the two approaches can perform differently over time.
Unknown older texture should be evaluated appropriately before it is disturbed. Do not assume material composition from color or texture.