Cabinet mfg will always cost more. Aftermarket you only get warranty from supplier and may lose mfg warranty on cabinet (check first varies)
My experience is I only do that as a cost saving measure on DIY so check with installer re labor and some do not like doing that.

bump

if you install rev a shelf roll outs according to their specs why would that void a warranty on a cabinet? But the cabinet manufacturer might have a more solid rollout tray and better glides, also various side heights on the roll out tray. the roll out chrome caddies for underneath the sink,I think should be fine chosen from assorted styles at hafele,etc,after the plumbing is set-because you wouldn't know how the space is exactly configured until all that is done by plumber,anyway.

Can't say why, just know sme do so check. Do lots of aftermarket under sinks, but whether factory or after I always specify the plumbing.
Other thing to check if not getting from whoever supplies the cabinets is who will be responsible for correct fit?

While most cabinet manufacturers products are more expensive up front, by the time you add in the installer's labor to put the accessories in, you will have spent more money and time than you would if you just ordered everything from the manufacturer. Also, many accessories, like lazy susans, are only installable while the cabinet is being constructed as the doors of the cabinets aren't large enough to get the turntables through after the fact.

My cabinet maker uses rav a shelf and he is a semi custom manufacturer.. He allows multiple interior manufacturers. I had to let them know which I wished to use before manufacturing began.