If you buy / build a mining rig, you can resell the parts after ROI, or even before with a loss,
if you buy an ASIC, and the ROI drops down, my guess is, you will make a bigger loss.
Buying your favorite alts directly might be the better investment, but I´m unable to look into the future.
Do your own math and expectations, and don´t risk more, than you can effort to loose.