Hello, Eugeny and Tatiana.
Horrors of Hell can be completed, but it has some challenging areas. For example, some tough enemies appear if the alarm is raised by the gate's guard. If you prevent the alarm from being raised, then the tough enemies won't spawn. You simply have to be more careful in order to win.
About
Project Uran, other than the optimization issues, there should be no problems with the addon. Yo$hik's anthology has some mistakes, including missing custom weapons that both
Project Uran and
Horrors of Hell use (the
MP44,
K43, and
PPSH). For example, there is a soldier on level 2 of
Project Uran who has no weapon on Yo$hik's anthology, but on the addon version by me all is well. Until these issues are corrected by Yo$hik, you will get a better experience if you play the addon version of my works instead of the anthology version.