what you must remember is they are only as good as your flow rate into the building , so if you have poor pressure and flow rate then they are not so good , but if you are only running a sink or two then , im sure it will be fine
Small undersink Unvented Storage Heaters are very good as they work under main water pressure so they provide independent hot water pressure up to 4 draw off points.( depend of model)
They could supply hot water to a sink or 2 sinks and shower etc.
Advantage of using them is that they required very small area under cupport etc and they are energy efficient too.
They only can be installed by fully qualified Unvented Systems engineer
I hope this help