So the move to unbound isnt a recent one, for me at least. Ive used unbound for DNS + DHCP for probably 4 or 5 years now, and the last version of this custom firmware used it as well.
As far as why I chose to go with unbound instead of the standard dnsmasq setup? Well, there are a few reasons:
Reason #1: Performance
When I switched over to unbound, dnsmasq had not-so-great performance. This was especially true with large adblock-provided blocklists (500k+ entries), where the performance with dnsmasq was borderline unusable.
Unbound handled (and still handles) blocklists with 500,000+ entries with virtually zero decrease in performance. When i last tested it the time for lookups increased by an average of 1-2ms with a 600k entry blocklist (relative to no blocklist), which is less than the "noise threshold" and not statistically significant.
Perhaps dnsmasq has improved in this area in recent versions....I really wouldnt know though.
Reason #2: privacy
Unbound is a recursive resolver - it directly queries the authoritative dns servers recursively until it can resolve the name into an IP address. There is no "upstream DNS server" that you are forwarding result to/from (unbound can be setup this way as well, but it isnt in my firmware build).
This has the benefit that you arent giving anyone else a list of all your DNS requests and by extension every website you visit. Sure your ISP could still pull that information out of your raw internet traffic if they really wanted to, but chances are they arent (unless they are court ordered to or something similar). I'd also guess that the logs of the raw internet traffic are deleted far more frequently than the DNS query logs, limiting how far back in time your ISP could theoretically make a list of your visited websites.
Reason #3: security
DNS servers that serve millions of people are natural targets for bad actors who might, for example, poison the DNS cache so that www.yourbank.com leads to an website IP that looks identical to your bank's website but isnt and is run by shady people in Nigeria. Unless you are personally and specifically being targeted, no one is going to go through the trouble of trying to do something like this when unbound has better-than-average security and is only serving people on your home network.
I think unbound is absolutely the best choice (by a fairly good margin) for anyone with a router that is similarly (or more) capable as the dynalink dl-wrx36 is (in terms of memory capacity and to a lesser extent CPU power and flash capacity).
For the devices that are closer to the "minimum openwrt hardware requirements", which (unfortunately) describes a lot of the routers that openwrt supports, there is distinctly less benefit. Unbound will be much less performant, since you might not have the flash space to compile in all the goodies (libevent, libpthread, etc.) and use the extra unbound packages (like unbound-control, which is needed to use unbound+odhcpd for DHCP). The cache would need to be toned down, as would how aggressive the recursion is for lookups. recursive DNS queries might take considerably longer than using a forwarding resolver here, and setting up unbound as a stub resolver you lose the security and privacy benefits mentioned above. In this situation, there is minimal benefit to using unbound over the lighter weight (and more established) dnsmasq.
Unbound is fairly lightweight, but if you only have, say, 16 mb of flash storage and 64 mb of ram to work with, the even lighter-weight dnsmasq is an appealing option.
First you tell me to look up myself, now you mixing up my written words? Good job.