I recently used a Teensy for exactly the reasons you gave, but I'm not familiar with Arduino's with ARM or FPGA. That sounds very interesting, do you have some links where we can learn more?
The "FPGA acceleration", on the other hand, is pretty limited. Arduino has one board -- the MKR Vidor 4000 [1] -- with an onboard FPGA. The FPGA can be configured with one of a couple prepackaged bitstreams to add I/Os or perform some specific video processing tasks; it does not accelerate processing on the Arduino MCU. The documentation for the board is minimal, and support for generating your own bitstreams is basically nonexistent. The whole thing has a half-baked feel to it; I'd avoid it.
I await the day when I can be like "nah, pin 2 will be a 16-bit 10MHz PWM output controlled by this register and pin 7 will be a frequency counter" and then access that with code on the microcontroller as if it was a built in peripheral.
I'm kind of skeptical I'll ever really get into FPGAs otherwise, my use case for them is always so simple, at the level of... well... PWM outputs and PID loops. But a microcontroller is fast enough for most PID loops so really to me the advantage is customizable function-pin mappings. I can't imagine trying to implement something like an ethernet stack on an FPGA though, although I guess you can run DOOM on one so maybe there will be a compiler from C++ to FPGA somehow that's not comically bad. ;-)
http://papilio.cc/index.php?n=Papilio.PapilioDUOHardwareGuid...
edit: yep...still for sale http://store.gadgetfactory.net/fpga/
The Arduino Due has been around for probably seven years and runs on ARM. There are many other versions now that are all ARM. The FPGA coprocessor one... hmm, I don't see it so maybe I just read it in the news or got confused by the Portenta that is supposed to be released about now.
The ESP32 one... I thought Arduino was moving to that for the RF capabilities but might be wrong, I can't find one for that either anymore. I know for sure you can get a ESP32 dev board and program it with the Arduino IDE and their Arduino-IDF libraries compile nearly Arduino-identical code with a few limitations (relative to the Teensy, which I think is basically perfect library API duplication).
