Functional programming if done well is much better for maintenance. I find a strictly typed functional code base very easy to maintain. It's far less a house of card.
Reading someone else's FP is like reading Perl, aka line-noise.
Having worked in Perl for years I agree that Perl is most often hard to read. I've also seen some horrific FP. Generally that's because it either FP stuffed into a language that doesn't support it or it's some crazy meta programming that you need a PHD to follow. Boring well written FP is much easier to read IMO than most OO.