LoRaWAN and LMiC are complicated enough of necessity, that the first question in choosing a platform generally needs to be “is someone already actively supporting this device for this use?”
Based on that I would strongly discourage someone who doesn’t want to get deep into LMiC internal logic from trying to use an Arduino-based LMiC on an ESP32 as the architecture of timing is just too much of a mess by the time it filters through the Arduino layer’s pretending to be a very different sort of system than it is and gets down to the LMiC code. Unless a repo is explicitly claiming good support and capability, the fact that the code can build and transmit doesn’t mean it’s going to reliably catch downlinks.
In constrast, past posts here suggest that the native ESP32 port might make things clean enough to be reasonable and get the timing right. Given the native port would be exclusive to the ESP32 anyway, it is unique challenges are presumably not going to be shortchanged there.
At any rate, there’s a lot of complexity there that there isn’t with the ESP8266 (and the more extreme ESP8266 pin shortages are with the smaller modules - better modules expose a reasonable number of pins, especially as most IoT sensors are bussed devices anyway) . The ESP32 is not the “better ESP8266” that many first assume, it’s actually a quite different part with a quite complex software stack.
There is no simple answer to what would be good alternatives for LoRa32u4 because that depends on one’s use case(s) and requirements.
A good choice would be the hardware whoever maintains the LMiC branch one wants to use, is focusing on for their own development and testing. For example, for MCCI’s LMiC that would be the (ARM based) Adafruit Feather and their own (Murata module) STM32L0 board, and to a lesser extent the ESP8266. In comparison the ATmega32u4 gets little if any attention there, presumably because it’s short enough on resources to not be worth investing effort in (though as a mature Arduino port that works the way Arduino code expects a platform to, presumably it does work to the degree that it fits)