Vim requires a certain version of python, but does not clearly document this fact.
A straight-up install of vim on pc without python results in the message "ultisnips requires py >= 2.7 or py3" EVERY TIME you open the program.
Many users will not install python at all without a clear directive. Those that do will naturally install the most up-to-date stable version which is not the version vim supports.
The pc install package should bundle the correct python library and make any appropriate changes to paths and environments to make sure it is used if it is required. Either that or the dependency on python should be removed - if python is not there then ultisnips should not work but it shouldn't generate an error message that users have to be bothered with.
At the very least (and this is really insufficient) there should be additional steps documented on the download page to explain to users that python is required and what version of python needs to be installed.