If you can afford them, these are great:
I have these and don't use them often. The ratchet ends are dodgey in my opinion. Like I could pull hard enough to strip them. But the box ends are fabulous (and tiny). I have these and the 15 degree DBEs and I don't feel they are interchangeable. These are longer. The 15 degree models I personally feel are more useful.
In use, I feel these have a pretty narrow use case. If I have access, I'd just as soon use a socket and ratchet, even tho these are really better/easier. Where these really shine is for access restricted work. Under a transverse mounted engine for example. Threading these up into a tough to access fastener. In that scenario, sometimes I almost can't touch that fastener (maybe because I'm on a creeper). So flex head would be hard to get seated. You can kind force these home, if you get my drift. The flex heads just flex out of the way. Snap On does flex well. Flex heads should be really stiff.
I think my advice would be to pass on these. I would instead focus on good quality long combination wrenches. If you have that sorted, go to the Snap On style double box ends. If you have that sorted, I think I'd choose the EZ Red/ Mountain/Whatever they call the long Taiwan Double Box ratcheting flex heads, even if they are spline drive. You can buy a set for the price of one of these. I really don't think you'll be using these regularly enough to justify them.
Last word, strongly recommend against choosing flex head wrenches over fixed heads. Also, don't choose ratchet wrenches over fixed heads. Nothing beats a high quality combination or DBE wrench. Nothing!

Wrench drawer in my main box. My combos are the Snap On XLs. You can kinda get a sense for the different lengths.

Wrench drawer in my service cart/guest box. These are normal length SOEXMs. You can compare lengths here as well.
10-12, 12-14, and 14-17 DBEs are really designed for JIS cars exclusively.
I'd like a 10-13, but I don't think they make it. 10-11 is a waste as is 8-9, in my opinion. Should be 8-10, 10-13, 17-19 (I think they make that one). 16-18 - not sure about that. I'm sure they do that just to avoid the ire of GJ's skip police.