Group training is unquestionably "easier" in the sense that running X miles at Y pace is easier with a group than alone.
However I still prefer training solo. I have complete freedom to adjust my training as needed, which is critical for me but others might not be as needy as I am.
Also I like to think that it builds a bit of extra confidence for races knowing you've put in the work by yourself and haven't even seen what you're capable of in a group setting. For example, this year I tried an 8k time trial on a track, solo, to get a feel for the distance. Ran 30:42 and was pretty much maxed out.
First 8k XC race was then a 27:15 on a somewhat hilly course with 200 other people just a month later.