You can design filters with 1% FBW using ustrip, but you will have very high insertion lossess and rounding in the passband, due to the limited Q of the resonators.
The unloaded Q of a resonator is related with the lossess, in the case of ustrip the losses of the conductor (gold, copper...) and the dielectric (alumina, whatever...). The width of the ustrip resonator is also related with its unloaded Q (more width, less lossess, higher Q).
To design low FBW filters you can use for example hairpin. There are a lot of books about rf filters (Pozar, Lancaster...).