Biscuits, doms et al can help with alignment. On the other hand, if you get their placement just a bit wrong, they make alignment impossible. Some biscuit joiners are not known for their precision and can introduce mis-alignment with every use. (You get what you pay for).
Some timbers don’t glue that strongly - the oily rascals such as teak, iroko and similar can fail to absorb glue if they’re particularly oily specimens. In this case, biscuits and dominoes can help keep the boards together. They do it more by swelling in their slots than via the glue, as the swelling tightens them immensely in those slots so they act like permanent internal clamps.
Alignment via cauls is certainly possible. But it complicates the clamping & gluing process as the cauls as well as the boards and the clamps all have to be aligned properly. The best cauls are slightly bowed so the convex bit in the middle clasps the boards first and the clamp at either end gradually presses the whole caul down on the glued-up workpiece. So you have to make the slightly-bowed cauls …
Rather than buy a biscuit joiner or domino just to help with alignment, why not buy a plano press instead? I used one of these for many years before having to sell it as a house move meant no room for many of my former tools. The plano press is the ideal tool for gluing up large and small flat items made of several edge-joined boards. Not cheap … but less than a Domino; and no expensive add-ons to tempt you either. 
Another alternative, requiring you to make those cauls:
I have some of these and they work well - a combined clamp and caul, really.
Lataxe
