If you change just the outside unit, you have the following issues: compatibility of the old A/H and compressor; newer unit will be a higher SEER and may further complicate the compatibility issue; line size on the new unit may be larger due to new higher SEER designs.
To me the issue is always the age of the inside unit. If it is 5+ years old, sooner or later the motor will go, usually there is a rust or corrosion issue with the old coil, and the coil is at an age where future problems could creep up on you. It is usually better to replace both under these conditions. Only if you have a real cash issue would I consider keeping an older coil, and then do some homework on matching and make no promises.
Since a new A/H adds a chunk of cash to the bill, with duct changes, etc it is a consideration, but as I said usually it is a consideration to keep the old one for a cash strapped retiree or similar situations.
The implications of only replacing the outdoor unit are that you can lose up to 40% of your capacity and efficiency. If you are talking heat pump, the variance in refrigerant charge is a factor also between seasons. A TXV installed in the coil can compensate somewhat for the variance in cooling systems, but if it were a heat pump, forget about it.
Sound installation practices is the key to success. Equipment is only as good as the person installing it.