I teach the Duke of Edinburgh Gold Expeditions and for the first time in 30 years we had an outbreak of Crypto on one of the groups while in the Welsh Mountains, which caused the students to be ill for two complete weeks. This could have been because they failed to boil their water properly as instructed (they ran out of gas) and didn't use tablets, but it can also be picked up from the ground. Either way, they were very ill. The severity may also be because they were fairly young (16-18) and hadn't got any sort of immunity.Always a difficult subject as some people are more sensitive than others to contaminants. I prefer the simple milbank and boil approach and have often just drunk the water from rivers untreated. Cryptospyridium and other cysts normally only give yu a 24 hour bout of 'spanish tummy' and then leave you alone. Some may find it seriously debilitating though. The last time I had it was over thirty years ago when drinking untreated from the Medway in Kent when I had no kit with me. Heavy metals I don't worry about too much. In my youth the water was piped to my house in lead pipes and we weren't so careful with the others as we are now. And I suspect the vegetables you eat may have their fair share in them anyway.
It's really a case of each to their own I'm afraid. But while custom filtration systems are probably good, they are too bulky and heavy to take on a serious walk - and the filters will probably run out just when you need them.
Like many others, I have used Millbank bags and boiling and have never had a problem - this year I spent two months on an expedition in the Amazon jungle and found this means very good - although people often forget that to get the best from a Millbank bag you need to brush it hard whilst it is wet on the outside with a stiff brush before it will give its best flow. We had to purify in excess of 150 ltrs per day for our team and used the largest milbank bags to remove sediment, and then aquatabs to purify it - this could then be used for cooking or drinking cold.