[Tomte]

I haven't found an iOS app that allows good search in plain text files.

About every app out there uses Apples standard search control, which finds files where text occurs. Not the line(s) with the occurrence itself, no forward/previous, only a list of files that contains the text.

Is Ulysses better in that regard?

[martinlillebo]

Maybe you'll like 1Writer for iOS. It lists files including the line you search for, and it supports the "YYYY-MM-DD Note title" syntax mentioned in the commment above. So you can point the app to the folder where you store your plaintext files.