All of what you said is true for any product that claims to offer an instant recovery feature. Any system will be constrained by its hardware, and it's never a good idea to put all your "eggs" in one basket. But since we're focusing on instant recovery here, we need to consider which appliance does it better.
There are some key differences Unitrend's approach that puts it at a disadvantage when compared to Quorum. When you begin to allocate space for instant recovery with Unitrends, you have to decide how much of your available resources you're going to designate towards instant recovery ahead of time. This has two big impacts on the Unitrends system. First; if instant recovery is a big priority for you, you're going to have to purchase a much larger (i.e. expensive) appliance to accommodate both the standard backups, and the addition of the instant recovery backups. Such is not the case with Quorum, since the focus of their appliance IS instant recovery.
Second, we were told by Unitrends that if we didn't designate and allocate space for the servers we would like to be instantly recoverable prior to a disaster (or even a single server failure) we could not do so after the disaster or server failure. Quorum reverses the approach, since the servers are instantly recoverable by design, you have to choose not to have them built automatically. Even if I set my server(s) not to build automatically, In about three mouse clicks I can tell my Quorum appliance to build the server on demand. I ran into this scenario in our last DR test, and in about an hour the server was up and working normally. To my knowledge, this would not have been possible had we had a Unitrends appliance.
In my opinion, Instant Recovery (as Unitrends calls it) is what Quorum does best. It is at the core of their approach to getting the systems they protect up and running as quickly as possible. Unitrends may have a similar feature, but it's just that, a feature, not the primary focus of the appliance.