Tradesmen have always been subject to prosecution for shoddy workmanship, and a number, more often gas fitters or electricians, have received custodial sentences when it has been demonstrated that their acts or omissions have directly resulted in a death or severe injury. Personally I don't see any reason why fire risk assessors should be any different. However, there are a couple of fundamental differences between the working practices of tradesmen and those of fire risk assessors which would make it difficult to directly link a death or injury to an act or omission of an assessor.
For a start, there are prescriptive tests and checks that tradesmen are required to carry out on any installation that they work on. And, trade bodies and BS documents produce model report forms to ensure that all tests and checks are recorded in a standard fashion and that any parts of an installation that are not checked or tested are clearly highlighted to the customer. Therefore proving negligence is a relatively simple process.
The process of fire risk assessment on the other hand has been promoted pretty much as a DIY task. Available guidance documents don't provide any consistency of approach or presentation, there is no clear definition of what constitutes a suitable and sufficient risk assessment, and assessors can rarely obtain the detailed historic information regarding a building that is necessary in order to make a full and proper assessment. (Ask your clients for a copy of their buildings fire strategy document, or any BCO approvals for building alterations or change of use and watch as they enter what can only be described as a trance like state)
So whilst I think that fire risk assessors should be liable, in practice, and until we move towards becoming a proper profession with clear definitions established in the courts, I suspect that it will be difficult to successfully prosecute fire risk assessors.