Disposable tattoo gun tips, by and large are considered to be much safer than reusable steel tips. One of the big risks with steel tips that can be re-used is that if they aren't cleaned and decontaminated properly then traces of one client's blood may be left behind on the needle. This can be a health risk, especially if you use that needle on a new client and mix their blood together. Additionally, it can be dangerous for the artist who gets pricked by the needle if it isn't cleaned just right.
Disposable tattoo gun tips are cheaper up front than re-usable steel tips. This is because the construction of reusable tips is meant for the long haul, whereas disposable tips are meant to be used once and then discarded. However, if you have to keep buying more disposable tips over time then they are going to be more expensive than buying a much smaller amount of re-usable steel needles. Some tattoo parlors will have both on hand and leave the choice up to the patron.
All tattoo gun tips have to be disposed of as medical waste, whether the tips are single-use disposables or reusable steel tips. All tattoo parlors need to have medical disposal boxes to put all tattoo-gun tips in before they're thrown away. Additionally these tips need to be taken to special disposal services since they can't just be thrown into the trash where they could pose a danger to waste-management workers.