Obsolete.
As they say, "if it ain't broke, don't fix it".
I switched a few years ago, maybe i wasn't using all of cscope's strengths?
The reason i like it is because it is implemented in the classic "Unix Tools" manner i.e. small, fast, self-contained, good feature set and easily integrated with many editors. Now of course you have lots more tools with similar functionalities but i find most to be too big and unwieldy with lots of dependencies and a pain to setup. If you have ever been forced to work on minimal systems with no Desktop GUIs, remote targets etc. you come to appreciate small powerful tools which you can carry with you everywhere.
Not in my experience. YCM does browsing in Vim better than cscope's the way i used both. It was an easy switch to make and not look back for me. YCM features seemed to me like a superset of cscope Vim integration. Maybe i was missing something? Jumping to and back to function callers, definitions, variables etc. Clang just seemed to work better. Possibly because it understands C parsing which cscope kinda does some heuristics and hopes for the best? Dunno.
But yeah because it parses properly you get tab completion that works too. That isn't a negative for mine. Don't have to use it, super convenient and fast sometimes when you do.
I run Vim in a terminal. Works most places with my vimrc etc that i would edit source code. Includes a mips router, arm phone etc.
I used cscope for years. Before that ctags. I consider them obsolete for my uses, but i may have been missing something. Still don't know what.
For example, for my work environment i have standardized on GNU Screen + dvtm + vim (few plugins) + cscope + ctags; thats it. Depending upon available hardware and need, i may add more tools but the above is what i try to carry everywhere.