Typically . . .
Low value properties (perhaps those worth less than £100,000)
Leasehold properties with only a short lease left on them (often less than 90 years)
Sheltered housing and other properties with an age restriction upon the occupants.
Properties with 'non-standard' structures (such as timber-framed or thatched buildings).
Properties with a history of subsidence or in an area of coastal erosion.
Properties with a history of flooding within, perhaps, the past previous 10 years.
Park homes.
Sometimes ex-council (or ex-housing association / ex-MOD) properties.
Properties used for commercial purposes (such as B&Bs).
[List based upon about half an hour's googling and reading loads of stuff from equity release providers, such as this
https://www.hodgeforintermediaries.co.uk/wp-content/uploads/2017/12/HL_LM_PROP-Property-Eligibility-55-1.pdf?v1 ]