There are two problems, even apart from quality, with 3-in-1 machine types.
First, is the limited utility and rigidity of the mill portion, even when compared to a mill-drill.
Second is price. Smithy and Shoptask want fairly high dollars for fairly marginal machines. If you can get one of these machines very cheap, it becomes a better prospect. I picked up a Smithy dirt cheap ($300) like new and with all tooling. I'm actually pretty happy with it for what it does -- cut odd jobs, do metric threads, and every once in a while be just right to turn something and drill or mill a feature on the turned part. I use two other lathes for what it doesn't do (hold very tight accuracies, cut pipe threads, feed the cross-slide, quick-change gears, etc. etc.). Threading is a disaster in most of the older 3-in-1's withut a halfnut -- using a VFD to slow things WAY down or cutting from spindle to tail are possilbe workarounds.
A neighbor has a ShopTask. I think the Smithy has slightly better finish and a slightly better mill (it cranks up and down for cutting things like keyways on shafts). He thinks the ShopTask has more nifty CNC add-on gadgets. Realistically, this is a comparison of two homely sisters. Not really that much difference. The Grizzly is probably about the same, the Harbor Freight probably has the most variable quality of all.
Something like the Emco is a much better machine, but unless you luck out, it will cost significantly more than a better separate lathe and mill.
When space is truly a concern, like machine shops shipboard, tools like the VersaMill were sometimes paired with a toolroom lathe. This is actually a pretty nice and versatile setup (though somewhat clumsy) but it rarely makes sense unless that extra few square feet are truly unavailable.
Consider, too, the option of a nice lathe with a milling attachment.
