1) You can't have an 'unlimited' amount of fluid like you can when sucking from a random container. It also might be difficult to just use a little bit of fluid.
2) A compressed tank has the potential to be far more dangerous.
3) The end device is more complicated.
Quit frankly, all we'd really need is a bag placed high enough and a controlled valve to control the output. Though that setup would have difficulties if only using small amounts of fluid.
If you need more capacity, stack more cylinders together. If you seriously need more than ~2.5 liters, then buy an old stainless steel water fire extinguisher off of Ebay and do some plumbing. Their tanks hold 2.5 gallons and are relatively inexpensive for that volume in somewhat body-safe construction. However, be aware that stainless steel does not always take kindly to chlorine or bleach. If your goal is to handle small quantities of liquid, then tie a rubber band around a syringe, attach a valve, and call it a day - skip buying other hardware entirely. Alternatively, put a bottle inside of the pressure vessel and insert a pick-up tube for a quick-change setup.
Safety valve set to 15-20PSI, rated working pressure of 28PSI, wall thickness of what looks to be around 170-220 mils. Dangerous? A typical soda can rests at around 36PSI at room temperature and has a nominal wall thickness of around 4.7 mils. By that logic, every refreshing canned beverage out there should be treated like a pipe bomb. Even a successful pump-based setup is going to generate some respectable pressures by necessity when handling a viscous fluid like we are: The problem we started with in the first place was not having enough pressure.
Are electric peristaltic/gear/centrifugal/piston pumps simpler to clean after use than a hollow cylinder? If you argue that people are interested in this device only because cleaning a syringe is too laborious for them, then it follows that this whole idea must be an exercise in futility: Any design we come up with is going to require cleaning in some fashion. That is the nature of the 'body-safe' requirement.
- The maintenance required for a stored pressure system would include removing the cylinder and washing with a sponge, followed by flushing the wet-side plumbing with a bleach solution. Small primary (AA) batteries will need to be replaced eventually if building a portable setup.
The maintenance required for a pump system would include drawing a bleach solution through the pump and plumbing for several minutes to ensure total eradication of residual fluid from the pump mechanism. If using an economy-grade DC pump, there will always be a reasonable risk of premature mechanical or electrical failure. It would not be economical to use primary cell batteries with this sort of setup, so secondary cells such as CR18650s must be used along with a battery management system and appropriate safety mechanisms if a portable design is desired.