My only experience of a unitary council is with Plymouth. In this case it made sense to separate from Devon giving local control rather than having everything run from Exeter. Similarly Torbay went on the same route. However a unitary authority has got to be big enough to properly run all of it services and initially there were definitely teething problems. Exeter now wants to have unitary status and I have my doubts whether this will work. From a council tax perspective I have lived within Plymouth and in the South Hams and cannot state that I have noticed a great deal of difference. Refuse collection, although fortnightly, is probably better in the South Hams than in Plymouth. I think that the ruling political party can make a difference as priorities will change.
As far as Cornwall is concerned I would have concerns about losing local representation and knowledge but some of the districts within Cornwall I understand are having difficulties employing staff with the right knowledge and coping with planning regulations etc. In Cornwall there may well be savings through combining functions centrally. It should not affect education, health, police or fire.
Hope that this helps.