I've used Heroku ever since I was a clueless teenager struggling to deploy my poorly written CRUD apps. Mainly because I didn't want to spend the time managing systems inside VPS'. Heroku also offers a free 2-year Hobby Dyno offering as part of the GitHub student pack which also meant I didn't have to bug my parents for their credit card.
Heroku led the PaaS infrastructure concept and is widely regarded as a revolutionary product. At its core, Heroku's platform runs on AWS EC2 machines and what is termed as Dynos, a proprietary containerization model extremely similar to that of Docker (developed before even Docker was created). Heroku also spearheaded the Git deploy architecture of rapid CI/CD.
Heroku's founding team and early hires were a tremendously talented group. Yukihiro Matsumoto, Ruby's creator, famously joined the company in 2011.
However, anyone now familiar with Heroku understands that its development has stagnated. After its acquisition by Salesforce in 2012, things have been relatively unexciting. None of the founders are still at the company and one of them has admitted that the platform has a few obstacles.
One of the largest complaints of Heroku is the prohibitively high pricing plans. The big cloud providers are almost always cheaper than Heroku. Non-hobby Heroku dynos cost upwards of $25/month for 512MB of RAM. An AWS t2.nano instance costs $4.75/month for the same amount of RAM. Throw in Elastic Beanstalk and you get your fully-featured Heroku clone. Google Cloud and Azure have similar competing products.
I wouldn't mind paying more for Heroku if it had a larger feature set than its competitors. However, the competition seems to be eating Heroku's cake. In recent years, many start-ups have been launched with the sole goal of replacing Heroku. Render, winner of the 2019 TechCrunch Disrupt Startup Battlefield, calls themself "what Heroku could have been". Although it's not quite perfect, I quite enjoy using Render and I find it a much better product.
With all said, there are still many fans of Heroku for the exact reasons I outlined. As one user pointed out on Reddit,
Another word for "stagnant" is "stable", IMO. I don't know that Heroku needs to jump on every new shiny thing to be useful.