Currently, getting the Safety Bubble perk cancels the special ability of the Shield Generator, which makes this less attractive for endless/dark castle runs.
An idea to remedy this is to have the Shield Generator extends the duration of the shield depending on the rarity - e.g. a Common extends by 1.0s, uncommon by 1.25s, rare by 1.5s, and so on until deluxe by 2.75s