I may not understand (although I think I do) but I have filled out the relevant schedules for taxpayers abroad many times. My first was in 1977, my most recent was last thursday, and most years in-between.
Planning and familiarity definitely makes filing easier. However, it's almost an impossibility for the novice and even many professionals, and it's necessary to keep up to date with all the new IRS rules.
A simple situation like a US expat with a UK employer sponsored pension into which the employee contributes more than their employer will probably lead to the need to file foreign trust forms along with PFIC and a mark-to-market on an annual basis, although there is language in the treaty that could be interpreted that the gains can accrue tax deferred so you should probably file an 8833 to make the treaty claim. I say probably because tax professionals differ on the interpretation and there is no IRS ruling. So you now need to read and understand the US/UK Tax Treaty which is wonderful bedtime reading and I can show you at least one Article in it that is regularly mis-interpreted by the IRS. In fact the explanatory notes to the treaty say something completely different from the Article in the tax treaty. IRS agents and 99% of enrolled agents are not familiar with the tax treaty, the nature of foreign pensions and accounts or how they interact with US taxation and of course you need to go through a similar set of laws and regulations to pay tax where you live.
Now throw in the out of phase UK and US tax years and the situation quickly spirals into a Kafaesque series of forms, FTCs and offsets. If the innocent US expat then decides to invest in a UK stocks and shares ISA they are in for a nighmare of forms and draconian tax rates and notice I haven't even mentioned FBAR and FATCA which are just informational forms that add to already complicated paperwork. Most US expats do not comply with IRS filing requirements and the IRS did not previously enforce the rules with much conviction. But that has changed and once people start filing FBAR and FATCA the IRS will start to ask "where's that 3520, or 8833 etc".
Next you have to deal with UK taxation. If you have UK income it will be taxed at source, but you need to know how much you've paid to get a US tax credit. If you have US income, the UK will probably tax that on an arising basis, but will also give you credit for US tax paid, leading to "chicken and egg" scenarios.
My next question would be how does the US expat deal with the new UK NEST pension plan on US taxes........Many US expats who work in the UK will be automatically enrolled in this UK pension plan arrangement. Should they opt out, thus loosing the employer contribution? or deal with the IRS tax filing consequences?
Even if this stuff was easy it would still be more than an inconvenience because of the associated fines and penalties.