Pros
Flexibility, ability to hide (if that is what you want - but very strange for a small company), fully remote working if that is what you want. Senior management / founders are real visionaries but that is where it ends, unfortunately many heads of departments are out of their depth.
Cons
Almost no benefits in the UK, may not get the best line manager, flat structure does not allow for career progression, difficulty in being able to put forward your ideas, often no-one will listen and you find it hard to understand why you got hired. Often decisions already made so unable to contribute if that is what you joined for. Almost zero collaboration as not only it's difficult to do so fully remotely, but also if you end up with the bad line manager, you will be totally dismissed in your ideas and will be able to do nothing about it as there is no way to go above your line manager. No HR, but they have few really stroppy and not pleasant people (that are generally disliked internally but still kept on!) to fill in that function. Poor / non existent company culture - since everyone is remote. Poor work life balance for those who are conscientious. The above very much depends on the line manager.