"Bluetooth" is a specification for hardware (the radio specifications for transmit/recieve, which includes the radio frequency, but there are other engineering details that would be total gobbledygook to everyone here but other electrical engineers to get in to), and underlying protocol -- i.e. the agreed-upon procedure for exchanging data.
However, how reliably two devices communicate with each other over BT has much to do with the vendor's implementation, and the software in the application that is using BT.
For example, one app might retry 10 times after failing to contact a target device. Another app might give up after 2 tries. Depending on the problem, the first app might appear to be more reliable, because it succeeds more often in marginal situations (for example, the device is at the distance limit for BT, so the signal is weak).
So comparing the performance of the standalone receiver to an iPhone is really an apples to oranges comparison. Or more accurately, comparing a Red Delicious to a Gravenstein.
Also, the Dexcom receiver is a single-purpose, dedicated device, so everything about it, including the radio communications, was designed and tested to collect data from the G5 transmitter. When operating, the G5 transmitter is the only BT device the receiver knows about, connects, and "talks" to.
A phone is connecting to multiple BT devices usually, with very different behaviors. This vastly complicates the situation. The phone may be having to manage streaming audio to a BT headset for music or phone calls while at the same time trying to connect to the G5 briefly to take a sample. When you have many devices connected at once, things can get flaky.
For example, when in my car my Galaxy S6 is connected to 3 BT devices -- my smartwatch, the car phone connection, and periodic attempts to connect to the G5. It works most of the time, but every now and then I have problems.
This will improve as the technology matures. Right now, we're in the early stages of all this connectivity -- BT is being asked to do a lot more than it was originally designed to do; when it first came about, it was mainly a single-device-at-a-time connection.
So, over then next few years, these integrated solutions on smartphones will have issues the dedicated hardware (Dexcom receiver) don't. Each of us has to decide if the convenience of of easy access to the data at a glance is worth the hassle to make it all work. It will get better!
One of the hardest things for us engineers here is how limited our ability is to help our non-engineer friends through the limited medium of an internet chat board... If I could only get my hands on your phone -- and I don't even know iPhones -- I'm 150% certain I could figure it all out and get it working well pretty quickly.
Over this medium I am pretty useless for iPhones, as I have no more helpful knowledge than anyone else.