I'd recommend something like a couple $25 Quansheng radios instead. More reliability, more range, less finicky, more durable and about the same size as this add-on.
(For some reason the radios are showing as $30 right now)
Yeah I have 4 that are great 2m/70cm, which is what the included antennas are optimized for. Even better with a cheap Nagoya antenna and non-stock firmware.
Unfortunately people don’t realize that even if you install a firmware that allows down to 11m, even connected to a $1200 base antenna the power output will be in the milliwatt range while throwing off on harmonics.
There’s also the case of hardware inconsistency and fakes. When I hook up 4 of the included antennas to my VNA there’s pretty big variance, and I recently tested a fake Nagoya that was clearly tuned for air band and not 70cm/2m as advertised.
The Qansheng's that I've tested have been fine, first harmonic down 44dB, which is OK for the FCC.
However, if you hack the firmware and transmit outside the bands the radio's RF is designed to transmit on, then you will probably see all sorts of spectral weirdness.
Another issue is people were “testing” them using an RTLSDR which very easily gets overloaded and shows harmonics where there are none. Even my local FM station shows up on my SDR at frequencies I know they’re not actually transmitting on.
I think it's poor practice for the seller to advertise those simply as a "Walkie Talkie," as that Amazon link does, without making it clear that it is an amateur radio transceiver that requires a license with an FCC-issued call sign to operate. I wonder how many people buy a pair of those and then just start transmitting without quite knowing what they are doing.
Using GMRS requires a $15 license that has no test and is good for 10 years. But nobody ever gets the license. GMRS is constantly full of chatter everywhere you go. Nobody ever mentions needing to get a license. You see YouTubers constantly using and promoting "Rugged Radios" which are just rebadged Baofengs. They are sold by the thousands.