The only thing is they cost extra to buy separately (for those filters that don't come with inbuilt taps), and you have to allocate some space along the hose line to install them (the double taps are long, which can be abit tricky if your hose distances are short).
For the installation of the taps, usually most people will install them closer to the canister side, mainly so that they can better hide the taps away from view. You can also install them closer to the outflow/intake pipes too, just that they tend to become more visible when viewing the tank.
You can install the reducers anywhere along the hose line, their exact placement aren't so crucial.
For a secure water-tight fit, a 1cm or more connection join between the hoses to the pipes should usually be sufficient. If in doubt, cut the hose abit longer to allow for extra flexibility and positioning, try to avoid situations where the hose puts stress on the connections when moved.