Is this really the case? Are ISAs made of of multiple PFICs and it is in effect impossible for people like me to file because the costs of doing so as so high?
There are several types of foreign investments that require detailed filing for the IRS. Unfortunately, a stocks and shares ISA is one of those accounts. For Americans in the UK, a stocks and shares ISA has been termed 'toxic'. Others may be able to give you a better idea of what is involved for the form required, but from others who have been through it, it is involved. It's generally accepted that a cash ISA is fine and is treated as a regular savings account.
The pension plan may be a subject of debate. It very much depends on the type, and ones reading of 'accepted' practices.
The person you have talked to may, or may not, be attempting to take advantage of your situation. It is not unheard of for back filing in order to become compliant to cost £15,000 to £20,000, but it is not the norm. If you employ an advisor to solve this, and with 3 years of back filing, be prepared to pay £2,000 to £4,000 at least. It may be more, it may be slightly less. It all depends on the number of your ISA's.
For quick compliance, the current streamlined programme is the best available, right now. It means 3 years of back 1040 returns and 6 years of back FBARS (FinCEN 114). The FBARs are not a difficult problem, and you may do those yourself.
You may also wish to consider whether or not you want to retain US citizenship. Regardless, you will have to do at least 3 years of back filing (5 to renounce), but the type of acquisition of US citizenship (assuming you were born a dual citizen) may be to your advantage if you wish to relinquish.
There is an interesting passage in the latest Presidential Budget Proposal, but it is not law; and with the dysfunctional politics in the US, it may never be law. But for someone in your position, it would be an advantage.
A much more risky scheme would be to rid yourself, now, of the stocks and shares ISA's, and file going forward for 2015. This is a very risky and not kosher, and you would be opening yourself to severe potential penalties if discovered.