Complex sentence: Although FFP designs are not susceptible to magnification-induced errors, it's challenging to design a reticle that is visible through the entire range of magnification e.g. a reticle that looks fine and crisp at 24x may be very difficult to see at 6x magnification.
Split into simpler sentences: Although FFP designs are not susceptible to magnification-induced errors, they have their own disadvantages. It's challenging to design a reticle that is visible through the entire range of magnification e.g. a reticle that looks fine and crisp at 24x may be very difficult to see at 6x magnification.