Nickel will develop microscopic cracks (not generally visible) that will let ammonia-based cleaners into and under the nickel plating.
Because MOST guns have a copper base under the nickel, Hoppes and other cleaners that have an ammonia component, will get under the nickel and attack the copper, causing the nickel to flake away in those areas. Check with Bersa and find out if they use a copper foundation for their nickel plated finishes.
If Bersa uses a copper base, I'd avoid using anything with ammonia.
(I've got a nickel-plated CZ; happily, CZ doesn't use a copper under layer.)