I've been enjoyed SH3 then SH4 off and on for years and find it to be as difficult and engaging as I wish to get... Difficult to believe it's over ten years old, and thanks to the SubSim community maven modifiers it has had a life long after its demise as a mainstream game...
FWIW, preferring "onboard" games vice the Steam online route I purchased the CD version, installed it in a folder in my SSD F drive and a couple of years later added SH5 which upgraded SH4 to v1.5 that is required for most mods. I play it stock with the only mods being for the large protractor (a must have) and mouse dragging for setting sonar direction.
Overall it has run well and I play it four or five hours a week.