Eurocontrol (I think) has made a paper discussing the impact of Transition Altitude (TA) and the consequences of various TAs.
I don't remember where it is though (and I can't find it on skybrary) but if I remember correctly the conclusion was something like this:
Low TA, easiest for enroute, least risk of having diffrent QNH for various aircrafts in the TMA (and if you have it they're generally going to airports very close so the diffrence is very small).
Downside is that during certain pressures the approach unit lose several available flight levels and it does complicate the work a fair bit.
High TA, removes the problem with losing altitudes in the TMA. The down side is that you need to start with regional QNH, since you can have enroute traffic going on QNH all the way (short hop propeller aircrafts etc) and you can have streams to airports quite far apart all being at an altitude.
There were also a medium TA option in this paper but if I remember correctly they concluded it was just in between, there are still lost levels in the TMA (just higher up where it might be easier to work with but can cause a ton of problem aswell) and there may still be aircraft needing a regional QNH rather than the airport pressure.
If anyone else has read this paper and remember the location feel free to link it, I will google a bit for it later (or tomorrow, I'm quite tired now
).
I do by the way blame the terrible grammar and spelling on the fact that I've been travelling all day, it's not intentional