There were clearly engineering problems, I heard somewhere they dropped it and it fell to pieces. So they had to go back and create a physical latch to hold modules in, and have locked the screen, CPU, etc in for now.
The good news? They've done the amazingly hard work on the unibus, driver system (automatic runtime loading), and physical connectors. You can still put a processor in and get a high speed, low power connection to it. That means someone can still make a fully 100% modular phone in the future since the specification is open, and in the mean time the ecosystem starts making modules
Many hardware manufacturers are salivating at being able to release just their hardware component without having to add a battery, screen, UI, connectivity, etc, just give you exactly the function you want for a fraction of the price.
Processors: - An extra GPU (and extra batteries), turn a lower powered phone into a gaming machine. Use two GPUs for high power VR (one per eye) - FPGA, a whole new class of apps that can make use of definable silicon - ASICs, specialized hardware to accelerate processes, for example high powered computer vision right there on your phone. TFUs will become the new GPUs as AI takes off, now we can have them early on mobile - X64 processor, run desktop apps natively on your phone? Who knows
Sensors: - all sorts of medical equipment like blood sugar monitors, spectrometers, disease detectors, this will change people's lives and will definitely sell ARA's, not just for medicine but also for farming - motion detectors - night vision / thermal vision - wifi vision to 'see' radio waves - project tango, to map physical objects to 3D models (for 3D printing, AR or computer vision) - super high quality cameras - holographic lenses - geiger counter (or bio warning sensor) for disaster zones
Radios: - Software defined radio, have your phone listen to all the EMI around you to gain a 'sixth sense', now your phone can also communicate on any spectrum / protocol, eg to a model plane on a custom protocol - external antenna port, pull it out of your bag and gain ridiculous reception, or setup a directional antenna to communicate large files over 20-30km with line of sight (an app could automatically get your GPS loc, and show on the screen where other link receivers are to find out where to point it) - upgrade early to the latest protocols like 5G/wifi with custom modules - FM radio or internet radio, tv receiver
A secure enclave to store your encryption private keys, so even if your phone is hacked they won't gain permanent access to your data or bitcoin wallet
Extra batteries
Removable storage returns (finally!)
The biggest thing they've done here is show incredible leadership, to align the entire world's hardware industry around a validated specification for intercommunication with software, and get market penetration. No longer do hardware companies have to struggle to provide crappy software and UI's for their products, just plug it into an ARA slot and know that you'll get communication, power and a screen for free. They can even skip the UI and just publish an intent, so developers will even integrate it all for them. This all means highly reduced costs for consumers, and suddenly niche products become viable
So yes, yes I'm very glad they worked through the issues in the last year and are actually going to release this thing. I'll definitely buy one and although Apple would never compromise their industrial design, I'm hoping to see module slots appearing on every phone in the future