Connection strength vs reliability

The penalty to reduced signal strength is throughput only.  However, operating at long distances and/or very low signal strength you might see reliability issues, due to the time-variability of radio signals and noise.

The theoretical maximum capacity, C, in bits-per-second of a communications link is determined by the bandwidth, the signal strength, and the noise by the following equation:

C = B * Log2(1 + S/N)

where B is the channel bandwidth, S is the signal strength, and N is the noise strength. S/N is also called the signal-to-noise ratio.

Bandwidth is fixed at either 20 MHZ or 40 MHZ by the WiFi standard. So, if the signal strength drops, or the noise goes up, then the throughput must drop to compensate or there will be errors on the channel.

The WiFi hardware tries to adjust the link speed to stay under the channel capacity, C, as determined by the currently available SNR. That's why your devices further away show a lower throughput.

If you operate under the channel capacity, the link should be reliable. However, with wireless connections, both S and N will vary randomly over time. Noise sources come and go, people or objects block the signal, etc.  At long distances and low SNRs the hardware may not be able to adjust link speed quickly enough to adapt to the varying SNR, and you might see reliability problems.  In most cases, if errors occur the hardware will detect the errors and retransmit the data, but this retransmission lowers the effective throughput.

Your devices will also have lower power radios and less effective antennas than the Orbi, so the device will be the limiting factor. At long distances, your devices may be able to see the Orbi but not be able to link back to it.