You are right for Handover to be performed the BCCH should be in SI5 List. The problem occurs if there are two neighbours with that BCCH one is defined in the serving cell and the other is not defined. then it measures the BCCH and since the same BCCH is used twice in the vacinity there is chances of it being interfered.
Scenario 1. A has C but no B and C has B.
Now for a case where imidiate neighbours BCCH is not in the list then. In DT you might notice Twin HOs. Lets say Serving Cell is A
best B and second Best C
1st HO from the the serving cell A to the second best neighbour C as the B’s BCCH is not added.
2nd HO from the second best neighbour C to B as C has B defined in BA list and B is the best based on leavel and so a PBGT HO will be trigerred.
A does not have B and C does not have B.
For this case handover shall not be initiated towards B. Worst case lets say a new site comes on air with ZERO neighbours then it will not take any handover traffic.