There are certainly better carbs to work with now, some areas have improved and others need a bit of help. At one time the Holley carbs did perform well almost always, carbs were sold sized for the demands of the engine they went on. You didn't run an 850 on a 327, it was reserved for a big block. Instead you might run a 650 or 700, when you do that the idle/transition metering worked correctly. Idle screws usually ended up about a turn out, transition fuel had reasonably correct amount of fuel and duration of operation to transition smoothly into the main circuit. And the circuits were close enough to run they way they should with a points ignition. Thousands of carbs were manufactured and flow tested, because a lot of them had to run on production vehicles they had to be right.

Now everyone wants a bigger carb, to make the most power they can. And that's OK to a point, however it changes the timing of the circuits significantly. Where you might have to rotate the throttle 20 degrees to get the mains to start, the larger carb may have to go 25. That means the transition circuit needs to fuel longer or work done to the boosters to get the mains to start sooner. Cams are much larger now, lowing idle vacuum and this disrupts how it runs at low speeds. Add to that the raised idle feeds and that makes things worse. Another is larger t-slots that some of the companies insets on, that changes how much air is bled back at idle and WOT. Then there is all the emulsion air that many think they have to have, so now you have the carb too rich at lower speeds and too lean up top. Yes it's nice to have everything adjustable, but they sure find ways to make you work for it. Last are a couple carb companies that just send everything out stupid rich. At that point you are better off with a production Holley or Quick Fuel.

As far as a production versus a custom carb, there are a few things you can't get with a production carb. Now I can rebuild any, but when building from scratch there are things you can't get in a production carb that can make a difference in performance. Usually not big gains, but noticeable none the less. Using a baseplate with smaller precise t-slots. Using bigger N&S to lower pressure which cuts down on fuel foaming. Using boosters that provide a strong signal and quality atomization. Using the best gaskets available. Paying attention to throttle blade adjustment, making sure they open fully. Adjusting the accelerator pump arms to provide full travel and fit on the right starting point on the pump cam ramp. Then being available to help when changes are needed, because yes there is no perfect tune. You can make a carb flow a perfect amount of fuel for the air going thru, but the engine may not run the best that way.