Every positive review is looked at skeptically, especially in the presence of any negative review(s). (Though I'd argue any review should be looked at skeptically, regardless of its tone. This is the Internet. Glassdoor didn't even try to perform any validation that I am, in fact, a current employee.) There are negative aspects to working at Fastly that I have experienced, so hopefully that will inspire some confidence in my review.
It can be easy to overwork yourself at Fastly. Being excited about the product combined with the sheer volume of work there is to do to get the company to really succeed in the market can drive a workaholic to tears. I hear that things are actually changing to address this issue, but until something formal comes out, I think it remains a valid concern.
Compensation and benefits could be better for a company of this size. And while I've seen many people move around, having new roles carved for them, I've also seen many stagnate. Again, there's been talk about making career advancement happen without moving up the management ladder, but until that happens, it remains a concern.
I worked as a remote employee for the first year-and-a-half of my employment. This was difficult for a number of reasons, but I did feel like without being in the office, I was being left behind. I have moved to the Bay Area to work at the office and that largely ameliorated my concerns. I can understand how a remote person could find working for Fastly difficult. I don't know what the remote experience is like, though I know we retain several remote people across organizations. I'd encourage the company to continue to try to support those people to the best of their ability.
To that respect, working as a remote employee can be difficult here, and I wouldn't really recommend it.
While I don't think Fastly is cliquish, I can see how people could think that. While I think some of that can be solved by simply approaching more people, I also don't want to blame the victim. I would encourage Fastly employees to be more gregarious and inclusive in general.