Fast magazine changes, good long range accuracy and decent power. The 'modular' aspect is a fairly new thing.. and it's possible to over customize instead of purpose build one.
A purpose built AR can be a shorty or a varminter or a 3 gun ruffian. An AK is an AK no matter what you bolt onto it. (There's only so much you can do to accurize an AK.) But you really can customize an AR without being a gunsmith.
As for reliability, my Colt 8920 is by far the most reliable semi auto of any kind I have ever owned. You might NOT get that from a parts gun--but i think the reliability thing is ancient history.
A purpose built AR can be a shorty or a varminter or a 3 gun ruffian. An AK is an AK no matter what you bolt onto it. (There's only so much you can do to accurize an AK.) But you really can customize an AR without being a gunsmith.
As for reliability, my Colt 8920 is by far the most reliable semi auto of any kind I have ever owned. You might NOT get that from a parts gun--but i think the reliability thing is ancient history.