Is 5-fu for keloids safe?

Yes. Yes it's safe if used as localized injection. There's minimal risk of significant systemic absorption if using small amounts. Important to limit total amount of drug injected so no problem with small scars but if there are lots of big keloids covering the body then need to treat small areas at a time to limit drug exposure.
Unknown. The long term safety of using topical 5-fluorouracil on keloid scar tissue is not known. Many doctors do use it to treat and prevent keloid scars. It can work well. However, the data on long term safety is just not there. Ask your dermatologist.