After having one of those undersink systems, I can say they work, the water is very pure coming out, only issue being that it’s basically pure water - no minerals, no anything - some issues there with actual hydration since drinking pure water can kill you - look up hyponatremia.
edit: might not be the case - see reply
The principle is the same as for the more expensive systems, what you don’t get is
potentially an UV inline lamp to sanitize stuff
a pump to increase the pressure and increase efficiency/speed
maybe a storage tank (some cheaper models have like a 1-2 gallon storage tank)
not sure about this one but more expensive membranes might have a better removal rating - mine said 95% I think
If you have around 45 psi of water pressure on your tap, it’s good enough, but don’t expect to be bathing in the stuff, it takes quite a long time to get usable quantities and I think you get 1L RO water to 4-5 L ‘waste’ water. Higher pressure - higher efficiency and speed but check the pressures the membrane is rated for. Also if your water is chlorinated you need some activated charcoal pre-filters because chlorine harms the membrane.
Maintenance should also be done regularly to change prefilters once every 3 months and the membrane once every 9-12 months. And it can get slightly messy on cheaper models.