Well, the main issue is that secondary haloalkanes aren't so easy - so often it's best to plan not to use them in your synthetic planning.
Both forming a carbocation first (known as SN1) is slow as the carbocation isn't very stable and just direct attack of a nucleophile (known as SN2) is slow as the carbon centre is a bit hindered.
The main issue is that because both of these substitution reactions are unfavourable to some extent, you probably get a bit of both happening at all times. But also, elimination reactions to form alkene (by E1 or E2 reactions) also become pretty likely as competing reactions for what you want.