I think the buffer vessel issue is more to do with modulating output and the need to fit one being negated with inverter controls on the units which have derived from air con backgrounds. There are some situations where a buffer may be advantageous but generally speaking they are not necessary unless you are fitting an old fixed speed heat pump.

Flow rates depend on the application and vary a large amount, we monitor the flow rate on every system installed and even with microbore pipework feeding the radiators you can still achieve the required flow rate range (10 - 25 L/M depending on the ASHP), heat the house and defrost effectively, again it comes down to design. The majority of retro fits we come across use 15mm pipework to the radiators and on newbuild you wouldnt specify 10mm pipe if designing for a heat pump system in the first place so either way the buffer/ flow rate arguement dosent work.

The autobypass is to prevent temperature/pressure related faults and protect circulators in the event all TRVs are shut. It is a slightly unecessary feature i agree as if your TRVs were all shut why would you want the heating on in the first place, why not just turn it off? Simple solution, dont fit TRVs to the bathroom radiator and use that as the bypass.

I think the most influential factor with any type of ASHP in terms of its performance will always come down to the quality of system design and installation, people will always have preferances as to which brand to use and as with all things there are differences in equipment quality, this is really the underlying message behind the EST report.