The customer support rep job is an entry level job and the only clear pathways available are into customer success and backline support via the product expert role. People successfully transition to other roles, but compared to success and backline support, other transitions are much less direct and have a lower probability, meaning you might be forced to stay in customer support for much longer than you desire. Timing is a huge limiting factor with regards to non-traditional internal transfers -- you might be waiting for the perfect opportunity that you ultimately get passed over on. More on this below, but this is a greater risk because support reps are not paid well.
Depending on your background, customer support can be a fantastic entry point into a strong company. But for some, the choice will be between taking a support role for 40k instead of a position that pays a non-negligible amount more at a non-tech company. I believed that I would be able to ascend quickly based on working harder than other support reps, but the internal structures do not incentive outperformers, they reward time in seat. Despite being able to complete cases with rapidity while maintaining a high NPS score, I was not able to take on stretch projects due to my manager tracking "availability". While the operation justification for tracking availability made some sense, practically it meant that I was not allowed to work towards my next desired role during my work day despite exceeding the core KPIs of case count and NPS. This is not how a salaried role should be structured. Becoming more efficient at your current role is how you stretch into the next role at any other company. My development was throttled due to this dynamic.
Finally, you'll be paid below national market rates at every step of the way at Hubspot. For some people the pros mentioned above are worth being underpaid and experiencing stagnation, but I didn't believe it was a prudent early career decision -- although it might be a valid choice at a more senior position within the company.