There are no rights and wrongs but there are pros and cons to both and these do need to be considered seriously.
The main benefit to working as a solitary to my mind is the freedom it gives. I generally work alone because I dislike having the schedule of what I do imposed by other people. If I joined a coven that met every Tuesday then eventually turning up would turn into a job and something
that for me should be spontaneous and creative would turn into dull routine. Working alone gives a witch the flexibility to do what inspires her when she wants to do it.
When you work alone and follow a path of your own creation you can evolve your progress as a witch in exactly the direction that works for you. There is a danger that if you do work with other people (particularly when just starting out as this is a key time in shaping your ideas and identity) you will be influenced by the thoughts and practises of others. This could have a stifling effect or set you off on a tangent that isn't the path you would have chosen if left to your own devices. When working as a solitary no compromise is required, you can work exactly as feels right to you and evolve into the witch you want to be and not just the witch that fits best into the group dynamics.
Similarly with the fact that you don't want other people imposing their views onto you, you also want to avoid forcing others to accept your own beliefs. There's an old saying that if you ask 12 witches to define witchcraft you will get 13 answers... Each witch finds her own way through the world differently and what works for you may not necessarily work for somebody else. A strong personality can easily overshadow a quieter witch though she may have equally valid thoughts and ideas. In some cases group work can lead to a cult of the personality of the loudest member and this benefits nobody. Equally a witch who validates her path by getting as many people as possible to agree with and follow her isn't doing herself any favours. And in among this hoo-ha one wonders if anyone is putting any effort into actually practising the craft..
What you learn in a group can be rewarding but learning at a single pace can be frustrating. I'll give you a personal example, a very very good friend of mine started studying the craft last year. Six months in I asked what kind of results he was getting and it transpired he didn't feel comfortable enough to conduct spell work yet. It is right for him to take a good deal of knowledge on board before putting it to practical use whereas regular readers might be able to imagine that the Degu Witch jumped straight into spell work from day one. Different approaches but they wouldn't run comfortably together, I'd be bored stiff at his pace and he'd be completely overwhelmed with mine. We would both have struggled if we were part of a group together.
Working in a group whether it be a team or a coven naturally involves some element of social politics and or disagreements. It is human nature to find discord where there are groups of people. The downside of this is that if it not checked by a strong leader, the politics can overshadow the benefits of group practise and the negativity may affect personal practise. That isn't to say all covens argue all the time, some work together very harmoniously. I suppose the question in my mind would be that if a coven did work together in a single direction how much compromise of individual selves is having to take place...
When a witch practises alone she validates herself, usually by virtue of her results and what she achieves with her magic. There is a danger in a coven that this will change to group validation, i.e. you're as good only as the group think you are. I'm perhaps thinking of the various degree systems awarded by some traditions (which I tend to be quite dismissive of to be honest. I'm not a fan of rewarding people for knowledge which these schemes invariably do. I prefer to be judged by what I can do not what books I happen to have read... Though I do acknowledge that in some cases these paths do represent a real dedication both to the individual's progression and to their contribution to the coven).
Group validation can have two negative results, either individuals are held back by poor group perception (in some cases deliberate bullying) or individuals get inflated egos due to popularity and the opinions of others. This to me is alarming as it moves a witch away from judging herself by what she can actually do toward being judged by what people think of her which of course has very little to do with witchcraft.
There are practical issues with coven work of course. It can be hard to find like minded people who live locally enough for it to be practical to meet up. The probability becomes lower when you consider that you're hoping to meet not just other witches but witches with a similar belief structure to your own. It's even harder for non Wiccan witches as, particularly in the States I'm told, most witches in modern times identify as Wiccan.
So in general I'm not in favour of working with others but there is a flip side to the coin and I'd be misleading you if I didn't mention some of the good points as well.
Witchcraft can be a lonely business. Much of the world dismisses it as being immersed in fantasy with no basis in reality. It can be very isolating starting such a journey alone and a companion or a coven can provide a source of support and confidence to the starting out witch.
The early days (especially if you're a book follower like my friend) can be a bit dull and it can be fun to have someone to swap thoughts with. You'll probably learn at a faster pace in a coven as there will be experienced practitioners willing to offer support, recommend the best sources of info and demonstrate practical workings. You'll also benefit from being exposed to different ideas and practises which may be confusing but which will also likely give you a lot of inspiration for what will work well for your own personal practises.
And, it can't be denied, working with other people does pack more of a punch to your magic. I do occasionally work with a partner and have to say that what we pull off is often nothing short of spectacular. To get those results on my own would require a lot more personal effort on my part. I also find having a partner's steer on an intended spell very useful and our combined efforts and ideas compliment each other very well.
So I'm not necessarily against the idea of working with other people. I do believe a witch should be confident that she can work alone but if she does choose to work with others, providing those others are chosen very carefully, there can be something to be said for more than one witch stirring the same cauldron.