1. adjacent channels neighbours : it will cause a voice degradation during HO, and if it happens that the
neighbour cell > (serving cell + 9dB)
then you will face voice quality issues during the talking as well, not only the HO.
C/Iadj > -6dB (3GPP recommendations) (or is it -9dB ?)
Same frequency :
C/I > 12dB
So you need to have the serving cell with a signal higher by 12 dB than the neighbor cell using the same frequency. That can be difficult to obtain !!
And it is not possible to declare an adjacency between cells using same BCCH frequency (even if different BSIC). It is forbidden.