A hammer will be reliable if the user uses it correctly, which it's possible to do.
With bluetooth I have the most popular, widely sold smartphone and the most popular, widely sold sports watch. They both claim to be compatible. I'm using both precisely as instructed. And sometimes they just.... don't see one another.
This isn't user error.
I miss my Pebble :( Though the Bangle.js and other "just uses bluetooth LE" devices have been essentially perfect connectivity as well. LE seems pretty reliable.
But like I said in the comment above, if the phone/widget manufacturer fucks up the Bluetooth implementation (misuses the hammer) because they cheap out and buy the cheapest no-name Bluetooth chips they can find on the market instead of buying from established companies, and then farm out the FW and antenna design to the cheapest sweatshop in Shenzhen, Bangalore or Eastern Europe since cost cutting and crunch in the consumer hardware industry is rampant and everything gets nickel and dimed to fractions of a cent while being rushed out the door to meet Christmas sales season or some other arbitrary product release deadline, then the phone/widget manufacturer who sold you the device is to blame for the rushed and buggy implementation, not Bluetooth itself.
So to conclude, if your bluetooth phone and watch are buggy, then blame their manufacturers for improper Bluetooth implementation and validation and ask for fixes/replacements/refunds, as a correct Bluetooth implementation is possible to do if you hire skilled engineers ($$$) and give them time to perform validation tests (more $$$), but instead, most of the widget manufacturers(including car companies) cheap out, like I said before, and cut dozens of corners to get their product out the door quickly and cheaply so they can skim more of the profits.
I hope it's clear now.
Source: Former FW engineer for Bluetooth widgets
What makes complex tech components like Bluetooth any different?
Also, Bluetooth isn't a single product made by a single company that dictates the standard and has the golden implementation.
To have Bluetooth audio between your phone and you PC for example, you can have different Bluetooth chips on each device, each with its own different firmware, then the OS on your PC and phone has a different driver and stack that interfaces to the Bluetooth chip and then there's the antenna design which links the two Bluetooth chips via the air. All these pieces need to come together in perfect harmony for you to enjoy seamless audio between two device.
If any of the manufacturers fucked up the antenna design, or the firmware on the chip, or the driver/stack on the OS, then your entire experience can fall apart and testing/debugging these thousands of permutations of various different pieces is nearly impossible. That's why Apple usually goes their own route towards proprietary tech they can fully control over the entire stack.