It's the manual pump ones that do that. The vacuum pump driven ones are ok. The manual pump ones were banned when I did rework professionally. We used Weller DS801's which were pretty good (when they weren't clogged up!)
If you look carefully at a board after using a manual one for a bit, you will see lots of very tiny solder blobs over everything. They get into everything and cause trouble. The solder cools inside the nozzle and then when you re-arm it, they fall out before the thing has a chance to suck them back up again.