It depends on the training, obviously practicing fighting without a human opponent is limiting.
But wrt to OTHER excercises beyond kumite, I prefer to do it alone. Group excercises like kata or kihon in pace, are alway annoying as everyone except me is off pitch

I work better when my inner pace can drive, trying to align to an external clock tend to also feel out of harmony. It's like dancing, you need to have a match with your partner (weight and heightwise) or you will just get dissonance in every movement.
The initial challenge training on our own is having no instructor to plan for you, so you have to make your own plans. I train alot with just the bag, and often i have a plan for what do for each session, and when I feel I go home and learned something from the session even having no instructor that feels good. Taking control is a value on its own. I hate group stuff, when we do say 10 kicks of one sort, and by the end when we are supposed to switch, I feel like I still would need a few more round to perfect it. That is frustration.
So for me perfect training is (basic training, kihon, kata or whatever tou want) on your own. all neede is a room, a bag and sometimes perhaps a mirror. Then meet with other for kumite and get some bashing. Perfect mix for me.