I've used my TRX-1 on P25 simulcast systems for 2 1/2 years both stationary and mobile with very good results. Transmissions break up in certain locations, but that is mostly due to strong signal overload from nearby cell sites and affects analog 800 MHz reception just as much.
With analog reception, front end desense and IF filter "blow-by" caused by strong nearby transmitters can be discerned by ear, as can local interference, static, intermod, etc. But with digital reception we don't hear these things and tend to forget they still play a role. A signal is either choppy or it is not, making it difficult to tell whether the choppiness is caused by LSM distortion or one of these other ailments (or a combination thereof).
As has been pointed out in the past, the TRX-1's performance on P25 simulcast systems depends on the specific P25 system (design, tower spacing, etc.) as well as the challenges of one's location. Results vary from excellent to unusable.
Obtaining truly solid reception would require not only an I/Q phase detector to improve LSM performance (e.g. SDS100), but a more robust receiver as well. That's why commercial grade radios used on P25 systems have so much greater reception range than scanners... and don't need an attenuator! A phase detector alone can't solve the P25 simulcast issue if an overloaded front end has already compromised the signal.
For a more complete explanation of Whistler's DMR tracking, see
this thread. My DMR tracking is limited but I've not had any difficulty with it.