Since LabJack data acquisition systems are designed to talk to a computer over USB, Ethernet or WiFi, that is where they really shine. All the work to enable communication between the LabJack and computer is done for you. We provide an easy-to-use high level library and examples for pretty much every programming language. In a couple minutes you can plug in a LabJack and read/write any register from any language.
Here at LabJack we believe an Arduino actually more like a PLC than a LabJack. Arduinos and PLCs are both designed to operate standalone, although they do have or can have some ability to talk to a computer. LabJacks are designed to talk to a computer, although T-series devices do have some ability to operate standalone.
During prototyping especially it is ideal to work with a Linux/Windows/Mac computer. The power and flexibility of such a computer is unmatched, and they cost little. We see that during prototyping you develop with whatever type of computer is convenient. Then if you are moving to a higher volume phase, you could replace the big computer with a Raspberry Pi or similar SBC, ARM or industrial grade computer.
Let's say instead you decide to develop your system without a computer. You would connect all your sensors and actuators to an Arduino or PLC, and do your programming ("software development") directly on the Arduino or PLC. You will generally be forced to use whatever single development environment is supported on the Arduino/PLC. You will not have nearly the flexibility, power, and connectivity, that you would have with a computer.
Where does LabJack fit in the hardware universe?
Cost: Arduino has low hardware cost but high labor costs. Wide range of costs on PLCs, but they are generally similar to LabJack. For very simple tasks the labor cost for a PLC and LabJack would be similar, but for anything beyond simple LabJack is going to be much easier to develop and troubleshoot.
Number of I/O: All 3 have generally similar numbers of I/O. All typically have analog in & out and digital in & out. Digital in & out are similar on all 3, although LabJack and PLC are going to have more protection than Arduino. Analog in on Arduino is quite primitive. PLC has better analog than Arduino. LabJack has much better analog than PLC.
Programming Language: Arduino is programmed in C. For PLCs ancient ladder logic is the main way of programming, but some new PLCs have fancier programming support. LabJack supports pretty much any language on Linux, Mac or Windows. https://labjack.com/support/software/examples
Sampling Rate: Back and forth communication ("command-response") is less than a millisecond on a LabJack. Not really sure about Arduino and PLC on that one. LabJack also supports stream mode where you tell it a list of channels to scan at a high rate (up to 100 ksamples/second aggregate). Never heard of a similar stream mode on PLC, and imagine it would be difficult and more limited on Arduino.
Example projects: Lots of customers & projects for all 3. What most people find in these projects, is that most (by far!) of your time is spent in software. Writing software, troubleshooting software, running tests, and changing configurations. In fact, even if you are troubleshooting hardware you will probably do most of that through software. Do you want to do that on an Arduino/PLC, or the language of your choice on a full-blown computer?
Flexibility: I would call all 3 flexible tools, rather than single application devices, but a LabJack + Computer is the most flexible.
Hardware Robustness: Arduino is quite low level, so their robustness is reliant on what protection you add. PLCs are designed for industrial applications and should be robust, but I don't know about their actual stats. They do have some extra failure points compared to a LabJack such as relays being built-in and a "computer" of sorts being built in. We have actually been working on a failure study, and it is not done yet, but it was prompted by the fact that we did not have a single return of a damaged or failed U3 in 2019, with something like 60k total devices in the field. We offer a 5 year official warranty, and in practice take care of everyone beyond that.