[blasdel]

The modern instance families support Turbo Boost, which is the X% guaranteed and up to %Y extra best effort you desire. On many of the largest instance sizes you get P-state control which lets you "disable that best effort extra bit" — but most operating systems will by default use it to strategically burst some cores even more at the expense of others.

Many workloads really just need a significant fixed amount of memory, and only periodically need high-performance CPU. T2 instances are much faster than you'd intuitively expect, if your workload is very bursty you can easily beat C4.

The newer Unlimited model "just" removes the edge case of being throttled if you run out of earned CPUCreditBalance, so you no longer have to reason about its interactions with other scaling factors.

It also allows a whole new usecase: if your workload doesn't need much memory or high I/O but will utilize as much CPU as it can get, you can save big by picking smaller burstable instances with Unlimited.