ExpressLRS is the newest of the current commercial crop of 868/915mhz equipment.
the first release was sent out in january of 2021, so it is only a couple years old, but in those 2.5 years, it has now had its third major software update.
whether it was originally intended to reallocate Frsky hardware (which i feel is not the first use) or if it was basically a ground up rewrite of OpenLRS to support 2 things, it currently offers more features than any of the others. Depending on how you use it.
It offers "OTA" updates for both the TX modules and receivers, in that "true" ExpressLRS hardware (not repurposed frsky hardware) all offers wifi connectivity. you can literally boot the hardware in "wifi" mode, connect to it as an access point and see what version of the firmware it is, change some settings, and update the firmware by uploading .bin files.
The main purpose of ExpressLRS was to have a very low latency, very fast refresh performance LORA based r/c link. using the 2.4ghz version, refresh rates as high as 1000 a second are possible, and latency as low as 6.5ms.
the designers "borrowed" the CRSF protocol from TBS Crossfire. Since it is a high speed serial connection, that helps in reducing the latency. There is no other protocol for communication between transmitter and module. the same serial based CRSF protocol is used to communicate with the receiver and either a flight controller or a PWM adapter.
ExpressLRS therefore requires a transmitter with both a high speed serial port, and a firmware that is either OpenTX/EdgeTX, or something similar (Ethos or ErSkyTX) the module currently does not support PPM, but a few people that fly airplanes feel it would benefit the community if that option were provided.
the later two versions (v2 and v3) have offered "advanced" features to support "wing" devices, which benefits r/c submarine use. v2 supports up to 12 channels, with 4 being full proportional, and 1 mandatory on/off channel, and the remaining 7 can be configured with varying settings from 2 position on/off to 7 bit proportional (128 positions) which is enough for dive planes, especially if you have a pitch controller.
v3 of the protocol supports up to 16 channels to the receiver, but the data is multiplexed, and that mode is only available at certain refresh rates.
ExpressLRS is the only protocol that is open source, and therefore hardware is made by several 3rd party manufacturers. I have bought equipment from 5 different manufacturers, and it all works together as long as the hardware is using the same major protocol version.
for servo output, there is one manufacturer of PWM adapters that makes a series of boards that allow anywhere from 6 to 12 servos to be used.
for those that are more DIY oriented, a stm32 or esp32 or rp2040 board has the horsepower to decode the CRSF protocol without any additional hardware, and a simple program can be written to convert output to pwm.
so, the ExpressLRS 868/915mhz equipment is the least expensive, and is available from multiple sources.
it has the highest refresh rate, and the lowest latency of the three. features that really aren't that important to subs.
it also has the highest hardware requirements, needing newer computer radios with firmware that can "speak" the crsf protocol.
like TBS crossfire, it has dynamic power allocation, so it will only use the power it needs to keep the signal good. you can set the max power level.
due to the opensource nature, different manufacturers offer different hardware, but the majority use the same chipset, which maxes out at 250mw, and some limit that to 100mw. Different country regulations may also limit max output power.
ExpressLRS is also the only protocol that offers both 2.4ghz and 900mhz equipment, so you definitely need to pay attention when purchasing, making sure you do not buy 2.4ghz equipment.