Risks in bacterial phage therapy
The Eliava Institute in Tiflis (Georgia (a country)) has been at it since the 1930`s. A US company sells a phageproduct for desinfection of meat.

The risk of Phagetherapy is there, if the phage is ill choosen or contaminated with other phage it could introduce pathogenic genes to the target or other bacteria.

If those risks are eliminated there is very little risk in phage therapy aside from the fact that it may not work at all. Phagetherapy is most useful in regions where bacteria are normally found, in other places the immunesystem will make short work of it.

The faster and cheaper sequencing gets the more effective phagetherapy will become. Since a "bad" bacteria can quickly be isolated and a proper "phage cocktail" can be prepared. If we get to the point that we can sequence the whole microbiom of a person then i bet that Phagetherapy will be a very normal thing.

To your cancer question, i think its rather hard to apply it to cancer therapy except if u are looking to combat the cancer with a Intracellular bacteria...but then again. How u get the phage to it. 

A excellent read on this topic is the Book "the bacteriophages".