"25% of our host resources to run their agent is just ridiculous."
I assume you mean their Controller VM, which uses 4 CPU cores and 32GB RAM (depending on features), per cluster node. If that's 1/4 of your virtualization farm, ouch.
The complexity is real. It helps to be familiar with Linux (especially KVM) and cloud concepts like object storage and metadata. Beneath their shiny UI, they use Hadoop for storage and OpenVSwitch for networking. I'm surprised your firmware updates were that bad. If you take one node offline at a time, even if that node breaks, your VMs should be fine.
I've been using it at work for a few years, and like it enough to use it for my VM lab at home. It sure isn't for everyone, and the "hybrid cloud" mentality isn't a perfect match for regular VM workloads, but there's a lot to like. Their REST API is great for automating cloud-like jobs.
I hope this isn't just a cover to raise their prices.