Compared to the noisy/cramped/claustrophobic/smelly Voyagers used on the Manchester-Birmingham service, the class 350s used on the Liverpool-Birmingham service are pleasanter/smoother/quieter and have a larger capacity per coach.
Pretty much any sub-120mph EMU would have a larger capacity per coach than a 125mph unit like a Pendolino, class 180 or Voyager. However, on a 350 a larger proportion of that capacity will probably be standing room (due to the wide doors) and a 350, because it only has 2 toilets, fails to meet the Rail Delivery Group best practice for "intercity or inter-urban services" of "85 seats per toilet" - the Voyagers do meet that. Also, due to the lack of internal doors seperating the vestibles from the passenger saloons, the 350s might not be conidered pleasanter than a Voyager (particularly coach D on the Avanti ones with their window-aligned seating).
Emus are much more straightforward to operate than locomotive-hauled passenger trains. IMO, all of the latter should be withdrawn from Britain's railways asap to improve reliability and cut costs.
Operation of multiple units tends to be much-more straightforward* than old-style loco operation with run-round at termini I grant you, but I'm not aware of anywhere that still happens on normal National Rail services, other than the sleepers. The only passenger operations I can think of still using locomotives and coaching stock are:
- GWR 'Night Riviera' mark 3s
- Chiltern Railways 'Mainline' mark 3s
- ScotRail Inter7City mark 3s (IC125)
- GWR 'Castle Set' mark 3s (IC125)
- LNER mark 4s (IC225)
- TfW mark 4s
- Caledonian Sleeper mark 5s
- TPE mark 5s (maybe; I'm not sure of the exact withdrawal date and whether they have actually gone yet)
Assuming you don't really think having diesel engines under the floor is a good idea on the sleepers, I can't see anything wrong with these. Two of them are IC125 formations with a locomotive at each end and the other four have a driving trailer at one end so that the loco can push from the other and doesn't need to be on the front all the time (so operation is not really that much different from a multiple unit). Unless you plan to remove them without replacement, it wouldn't necessarily cut costs either because I seem to recall reading somewhere that loco+stock (with driving trailer) tends to break even with units once you get to five coaches and loco+stock then becomes the cheaper option if running more than 5 coaches. This means the GWR Castle and Inter7City 2+4 sets are possibly a little 'in the red' compared to a unit but the IC225s are nine coaches, XC's IC125s were 2+7 (not on the above list, because GOV. have stupidly axed them) and the Chiltern, TPE and TfW sets are all (or will be) at least 5 coaches.
* although the myriad of incompatible couplers means multiple unit operations are not as straightforward as they should be.
Something needs to serve Winsford, Hartford and Acton Bridge, and the demand and number of settlements solely between Crewe and Liverpool doesn't justify a dedicated stopper like it does between Crewe and Manchester.
There's Penkridge as well, so the stopper wouldn't just be between Crewe and Liverpool. Why not add a few more stops to the current Birmingham-Liverpool 350-operated service to make it even more of a stopper and extend one of the three Avanti Euston-Birminghams each hour to Liverpool as a fast?