first off, "Arduino" is both a piece of hardware and also a concept.
the concept is free programming software, a simple development board and a bootloader to take all the hard work of programing out of the project, limit the ability to interface with the microcontroller, but offer a very user friendly experience.
why an UNO or MEGA is not suitable ? there is zero protection on a pin.
24 volts, fried
short to ground, possibly fried
voltage spike, fried
emf interference, bad signals.
the list can go on. and there is a simple solution to each problem, and every poster so far on this thread can add another way to damage the pin, create a false reading or some other issues.
you can use an opto on an input to isolate the input from factor floor.
you can enclose the unit in a faraday shield, simple copper paint inside the plastic enclosure does wonders.
the point is, when developing on your bench, you only expose some risks, and even then, most of us understand what magic smoke is all about. on a very personal level.
if you look deep in your Allen Bradley PLC, you might find a chip that is just as fragile as the Arduino's 328, but you will find multiple layers of protection from the shop floor.
there is no reason you cannot use a simple board. as GoforSmoke said, a development board is great for the bench, bad choice for installations. pushing a wire into a friction slot is very risky, a compression slot that scrapes the wire is not much better although a little more secure, and a screw connection is vastly superior.
I like the nano plug-in screw terminal boards, but the plug in part is less than idea for the factor floor.
solder is the best connection, as are gold plated pins vs the lead ones.
rising cage screw terminals offer better connections than a screw twisting on a wire. the screw can displace the copper and twists to cut and deform the wire.
we can comment on almost every aspect of the physical board. then there is a the software, debounce vs. no. changing state from a 2.5 v input signal on a 5v digital pin....
There is no doubt that every single problem has been found, dealt with and available in some form or another.
there is an Arduino PLC, ✅ Arduino PLC, Raspberry Pi PLC and Panel PC, ESP32 PLC for automation
This is not to tell you no, but to let you know it CAN be done !
that $1,200 PLC has basically, the same heart as your $2.31 micro.
just has layers of protection.
and, if your project is dedicated to one thing, you can protect for the problems of that one thing and not have to be prepared for every conceivable mishap that a PLC has to address.