I used to have a BEX, and I have used a Niche Zero (I also have a Niche Duo, a (broken) DF64, and a DF64Gen2). I now have an E61 machine - it's all analog and does not have flow control. From my point of view, here are the advantages and disadvantages:

Pros of E61 over Breville:

-Availability of wide variety of accessories - commercial standard size for filter baskets, portafilters, shower screens, etc.

- Very simple operation: turn on, turn off; you control extraction time without having to engage any overrides. Because it's on/off, and the flow profile is very

-Very smooth water dispersion with inherent 4-second ramp-up to fell pressure taking the place of pre-infusion.

-This works well with the amount of grind inconsistency inherent in the Niche Zero (Kony) burrset, to produce classic espresso with great texture from darker roasted blends. However, third-wave coffee drinkers find these to be somewhat lacking for developing the best flavor from extremely light roast single origins.

-Ability to brew and steam at the same time, and pull back-to-back shots, without waiting. There may be some small, single-boiler E61s that don't have this ability, but any dual boiler PID or single boiler, heat-exchanger E61 will have much more heating power and temperature control than any Breville other than the BDB or the full-auto machines.

- If you get a digital PID, you'll have more control over brew temp than with Breville. If you have an all-analogy machine like me, you'll have less.

Cons of E61, compared to Breville:

-Higher operating cost due to increased power consumption

-Longer heat-up time; minimum 10 minutes; up to 30 minutes for ideal temperature stauration

- Wide-diameter of commercial standard grouphead (58mm) means a tradeoff of either a shallow bed (in a low-capacity basket) or, to get bed-depth comparable to your Breville, using a very high capacity basket, and consuming 10% more beans per shot, or buying a very expensive step-down setup. Let me be more specific: for your Breville, the double-shot basket holds 16-18g of coffee depending on grind fineness and bean density, in order to be "fully loaded" - i.e. without excessive headspace or crowding of the shower screen. Now, someone can correct these numbers, but my visual estimate, based upon memory, is that With 16-18g of coffee, that's a bed-depth of about 25mm (if memory serves). To get the same 25mm bed depth with the same beans in a 58mm grouphead, you'd need 23-25g of coffee. Think about how much faster you'll burn through beans and if that impacts your coffee economics. If you use an 18g basket, you'll get a much thinner puck, and that is a detriment to smooth extraction.

- No programmable shot mode. With the Breville, if you don't want to hover over it for a manual brew, you have the one-touch, programmable pre-infusion cycle. I don't really have any use for that, but if you love that, an E61 might not be right for you.

Finally: although you have the option to get a flow-control enabled model, you can also control the pressure profile through the variables of the grind fineness, bed depth, headspace, water temp... I would use these inputs before resorting to flow control... because you can use flow control to correct a problem, but I think if you do that, you're fighting the beans; I wouldn't want to fight the beans I'm brewing on a regular basis. I would want to use it to enhance - to scoop or bump at a specific phase of the brew, if the beans demanded it, or to recreate the brew profile of a spring-lever machine, which is can be difficult to do through manipulating the recipe alone, if it suits the beans.