Many of the combination machines incorporate compromises in their design that enhance neither lathe nor mill operations. It is really necessary to study and understand the machine, and determine if it is going to be suitable for the work you will be doing. Grizzly offers a 13x40 lathe with a vertical mill/drill mounted to the bed. It appears to be the least compromised design. Smithy might have something similar. Many combination machines incorporate neither a quick change box nor a threading dial; the absence of these features does not facilitate thread cutting.
In many respects, any machine tool is better than no machine tool.
Having separate machines is better than having a combination. But decent work can be done using a combination machine. I did pass on buying a combination machine in excellent condition for a very good price, simply because the lathe portion could not do the barrel work I wanted to do. I knew that I would need to get a lathe in addition. And once I bought a lathe, the milling portion of the combination machine was going to be quite light duty. So, I bought a good sized lathe, and a sound mill/drill, both used, when the opportunities presented themselves. I also have the space in my shop, with easy access, to place separate machines.