with-other-threads-disabled disables all the other threads (that is, the
mp:process objects), executes body and then enables the other threads. Thus it guarantees "single-thread execution" for the forms in body.
The point at which each of the other threads is stopped is not well-defined. It is always a GC safe point, but it can be inside manipulating some data structure or while holding a lock. As a result, if the code in body accesses a data structure or tries to lock a lock, it may see an inconsistent structure or get an error about calling process-wait when scheduling not is allowed.
As a result,
with-other-threads-disabled is safe only if the code in body does not do anything that accesses trees of pointers and expects them to be in a consistent state and does not use locks. Any other code may, rarely but not never, get some unexpected error.
LispWorks User Guide and Reference Manual - 20 Sep 2017