Sorry, clarification: Rejuv bead is better if you don't depend on potions, which a lot of junglers don't. WW currently has 8.3 health/5 With rejuv bead that beats 3 potions after just over 500 seconds or 8 minutes. Of course health regen increases as you level so I think it actually breaks even after about 7 minutes. In the 2 minute clear that restores about 100 HP.
If we compare to Rejuv is breaks even with the first two pots at 300 seconds. Of course rejuv potions regen so I don't think it is comparble to those since their regen average changes based on how much you back.
So 7 minutes of jungling makes Bead > Pots since junglers generally don't take biscuits. It also builds into items like tiamat. In regards to that it is also a 100% gold efficient purchase but we aren't worried about that.
Now most tiamat junglers can clear on refillable, that means that most junglers only need 250 health max in their first clear to clear safely. In comparison to that WW needs 150 less health on the first clear which as a sustain jungler designed for newer champions (who don't know how to kite jg camps) is very reasonable. Personally I would expect a new player to lose 200+ health compared to a knowledgeable jungler. In regards to that WW clear with rejuv bead by someone who knows what they are doing is completely reasonable.
Any jungler who can normally take refillable and survive their clear with at least 150 more health than they feel is too low to risk clearing (after the camp, which means the health at which you would give up, around 20-50 HP for me since you can leave after any attack.) That means rejuv bead functions just as well as refillable for clearing if you normally end a clear with 160-200 HP. Add onto this that smite health for missing HP so the initial hit of rejuv bead isn't as impactful as it seems. Refillable potion is more gold efficient than potions because they keep refilling. The reason not all junglers get them is because some need the extra HP boost of normal pots.
In the same way junglers who only need 100 extra HP in their clear can pick up Rejuv bead because it is better in the long term then refillable because it builds into other items you will have all game, and scales throughout the game too.
So while it is not better health wise for the first clear or two it is not much of a hit to take to get away with in return for healing throughout the game + getting items faster. Most junglers with healthy clears like Vi, Lee, WW, Fid, etc. can get away with it. For sustain junglers like WW and Fid it is completely worth it purely for the gold efficiency/not having to buy pots later or take up another slot.
Really any jungler with inbuilt healing is going to be obscene. Look at old Yorick, I would start with fairy charm for the mana regen and never drop below 80%. The jungle is designed to let pretty much anything clear through it. Those with inbuilt sustain are naturally going to go overboard because that is what they are meant to do.