Interesting.. since the U4 is 2.4G and the Senn/Shure units are not.
I would not consider the U4 to be on a par with them as it's far more susceptible to frequency issues.
If you had "squishy/noisy" feeds with the Senn/Shure models, I'd guess something was wrong in the setup, or you were getting interference somewhere, or using the wrong "band" for your location.
I've used the U4, Carvin PM900, Shure PSM200/300, Senn G3 and Mipro 909 systems....
Yes, the Senn units have a noise floor - but that can be tamed - and has no impact when audio is being monitored.
With the Senn/Shure units, it's also important that one scans for open frequencies (with the receiver) then sync with the transmitter.
PS: I've never used the stock buds..
And what do you mean by "squishy"?
When you're used to the squeaky clean, noise free operation of wired in ears fed by a headphone amp, the noise floor of those traditional UHF units is super annoying. I've never owned them, but was provided a set by the sound techs on various occasions. I've got no clue how they were setup, but I've definitely played the G3, G4 and PSM300 on several occasions. The PSM300 seemed unable to handle any signal without compression, even if it was just vocals. That might have been a gain staging/setup issue, but the tech at that gig couldn't get it out. The Sennheisers were particularly noisy on various occasions. The last time I got to use a G4 set it was so annoying I switched back to wired IEMs halfway through soundcheck.
In my experience those UHF units are in most cases inferior to a decent digital (2.4 or 5GHz) set. A big touring band has a dedicated wireless tech who makes sure all frequencies are properly set and who uses those flagpole antenna's to amplify the signal. But for most consumer/prosumer users I'd say the digital systems are way more solid due to it being digital.
The UHF system is like your FM radio, if another station is broadcasting on a different frequency that is close to the one your listening to, it might still cause interference. That's because the radio waves are "open", interference or other signals can creep in without knowing the source or the fidelity.
The digital systems are more closed off in that sense. As long as there is a strong enough connection between transmitter and receiver, there is not much that gets in the way of it. Only if you have the same set as someone else on stage and you set it to the same frequency, you will pick up what the other is transmitting. But even then it will only pick up 1 transmitter because they are digitally synced.
All the talk about the congestion on the 2.4Ghz band in this context is, in my opinion, nonsense. Simply because as stated above there is a digital handshake between devices so as long as my transmitter and receiver are within reach of each other it works flawlessly. And it even sounds better, which is fundamentally due to the 2.4ghz frequency allowing way faster transmission than the lower UHF frequencies thus allows for higher bit rates etc. (The same applies to 2.4Ghz Vs 5Ghz, which is why 5Ghz is the faster WiFi option in your home network).
I have never had to search for a free frequency and never had dropouts on 2.4Ghz, while with UHF systems I have never ever experienced noiseless operation. UHF is basically old tech. Just like in the petrol to EV transition, UHF still has its place in professional applications where extra long range is needed, but most consumers are better off with a 2.4 GHz set (or EV for that matter).
