There is nothing sacred about an ammeter. It is just a way to read the health of your charging system. The issue isn't the ammeter. It (danger) is, as described above, passing the entire electrical load of the car through a rather small connector in the dash, one that most people hardly know exists.
A voltmeter monitors system voltage and can tell you the health of the charging system if you care to learn a little. When the charging system is stable the voltage on most modern cars will be around 14.5. The voltage regulator adjusts the field current to maintain that voltage. When there is a heavy load the system voltage will drop as the alternator reaches the end of its capacity. This is quite rare except in the case of electric cooling fans which present a high load to the system whenever they are on. Lights are another big energy user that are on for long periods of time, although with many cars changing to LEDs this load can become very small. Big stereos may have momentary huge loads and steady state loads of up to 50% of the amp's capacity, in the case of Class AB amps (old school). Modern Class D amps can cut the static load quite a bit. My favorite electronic ignitions use 0ne amp per thousand rpm. I don't know amp draw of a fuel pump.

Any time you bypass some of the car's electrical load around the ammeter you are making the ammeter irrelevant. It needs to see the entire load to function properly.

IIRC the ammeter is really a voltmeter reading the voltage drop across a known resistance. But the problem isn't the alternator, it's the bulkhead connector. Removing the ammeter eliminates the need to run the entire electrical load through the firewall. When this is done, the easiest way to monitor electrical system health is to use a voltmeter.

R.