After putting more hours into this game, my opinion has since changed. This is one of those games where it's going to be more fun with friends, but you can get through most of the game solo if you don't screw up. However, doing the higher difficulty missions are very challenging solo, so I advise you if you decide to do them higher than hazard level 2 to get a skilled group of friends to do them with you.
It's also really slow in terms of what you can and can't do if you don't have one of the four classes promoted. I stuck with one class and this was somewhat the amount of time it took to level them up in order to get my Driller to promotion.
Also the new missions is fun.

After putting more hours into this game, my opinion has since changed. This is one of those games where it's going to be more fun with friends, but you can get through most of the game solo if you don't screw up. However, doing the higher difficulty missions are very challenging solo, so I advise you if you decide to do them higher than hazard level 2 to get a skilled group of friends to do them with you. It's also really slow in terms of what you can and can't do if you don't have one of the four classes promoted. I stuck with one class and this was somewhat the amount of time it took to level them up in order to get my Driller to promotion. Also the new missions is fun.