Overflow systems, internal or external are beneficial in that you have to use them with sumps. A sump is nothing more then a water storage container in which all filtration systems will be held, such as protein skimmers, filters, heaters, media bags, refugiums, auto top off controlers and especially your main pump housing.
A sump also increases the amount of water volume while hiding all of the equipment in an easily accessable place. You can also work on/in your sump without bothering your main display aquarium.
I believe they have more benefits than detriments. If you do not have a pre-drilled tank, you can always go for an external overflow. Beware, however, that you need a pump that pumps hard enough to make sure that it does not break siphon.
Good luck!