* There is no composable toolset, but rather one monolithic program that does only a small part of the job. envdir and setuidgid are -u and -e options to the supervise analogue that is that monolithic program, "immortal". All of functionality of the other daemontools tools, such as envuidgid and softlimit, let alone the likes of runit's chpst, is missing. Not only are there no composable tools for it, the monolithic program simply lacks their functionality entirely. One can extend this functionality with the composable toolsets, with tools to handle things from scheduling classes to control groups. The monolithic program lacks all of this.
* There is no equivalent of the service restart/finalization mechanisms of runit, perp, s6, or nosh; which permit taking various actions dependent from how a service program terminates.
* Nor is there an equivalent of the at-stop and at-start mechanisms of s6 or nosh.
* There is no service readiness mechanism, as s6 and systemd have.
* s6, nosh, and systemd all have the concept of "oneshot" services. immortal does not cater for them.
* The svscan analogue, named "immortaldir", lacks an equivalent to the "down" file mechanism from daemontools, runit, perp, s6, et al.. All services always come up at bootstrap. There is no enable/disable mechanism at all.
* The author hasn't borne in mind the design lessons from svscan, the reasons why it works as it does, such as only having symbolic links in the scan directory, not the actual service definitions themselves. There is a race condition between copying or saving the YAML file into its directory and immortaldir trying to open and read it, which can result in immortal reading a part-written configuration file. There is also the potential for hidden files to be recognized as services, something that svscan explicitly prohibits.
* The author hasn't borne in mind the design lessons from s6-svscan. immortaldir wakes up and polls every few seconds instead of being event-driven, which as M. Bercot notes is a consideration on systems that want to save power.
* immortal does not have logger services, as perp, s6, daemontools, daemontools-encore, freedt, nosh, and runit all do. Instead, it has only the options of the service manager itself writing the log file, or a non-supervised non-manageable logging command. It is thus not possible to control logging through service management.
* There is no equivalent to multilog's rotation scripts, filtering, multiple log directories, or automatic timestamping.
* The log rotation uses the old .1 .2 system. The author hasn't borne in mind the design lessons from multilog, the reasons why it did the things that it did. So it doesn't ensure that logs are safely written to disc at rotation time, for starters.
* The service management API is not compatible with daemontools, daemontools-encore, runit, and nosh; which all share a common binary-compatible subset (the daemontools service management API). This API is, rather, a WWW server. Where daemontools et al. send simple one-byte commands; this API uses JSON and the Internet Message format meaning that a simple command to send a signal is actually an entire HTTP/TCP transaction, with all of that parsing and marshalling going on, under the covers. This flies in the face of the "do not parse at runtime" dictum followed by the other toolsets. There is an entire HTTP parser and server running with superuser credentials inside the service manager here. Ironically, this is a criticism that people vehemently (but erroneously) level at systemd. This service manager actually does do this, though.
* It also contains an entire YAML parser.
* There is no service interdependency mechanism; at all. There aren't even the tools for the old "thundering herd" approach of starting all services and having them manually check their dependencies in their "run" scripts. There is nothing there akin to s6-rc or to the service bundle dependency mechanism in nosh.
* There's no mention of portability to the Hurd, Illumos, OpenIndiana, the Windows NT POSIX subsystem, the Windows NT Linux subsystem, and suchlike. Several of the other systems address the fact that "portable" to any "UNIX-like platform" means more than merely FreeBSD, macOS, OpenBSD, and Linux. s6 has notes on how to build it on Solaris, for example.
* The new and interesting thing is not the fact that it focusses on the solved problems of fghack and PID files, which ironically have been going away over the past decade. The new and interesting thing is spinning up service and supervise directories on demand from a YAML definition. nosh has the ability to spin up a service directory from a systemd service/socket unit file definition; but the other toolsets leave actually constructing the service and supervise directories from scratch to third parties. That said, such directories are simple enough to construct by hand directly, on the same order of simplicity as the YAML definitions here, in fact. It is notable that the YAML definitions again seem to have features weighted towards PID files and suchlike which are problems that are in the process of disappearing.
* One of the fundamental design concepts of the daemontools family is that the service manager knows the process ID of the daemon because it remembers it from when it spawned it and knows that it does not go away until wait(). immortal employs the old broken idea, that this replaced, instead. The code reads process IDs from PID files and tests whether they are valid by sending them signal number 0, in the race-prone and risky way that the world has known not to do for decades. Witness https://github.com/immortal/immortal/blob/9ed9087a8ef1e64d64... and https://github.com/immortal/immortal/blob/9ed9087a8ef1e64d64... .
There is a lot of prior art here that this design is just ignoring. Overall, it is a retrograde step, to a system that is less capable than runit or daemontools are, is less robust, and is less well designed. A better approach is, if one wants to drive services from a WWW browser and to create service definitions from a configuration file, to layer a WWW server and a service definition creation tool over something like s6, which is more complete and more widely portable than immortal.
I also recommend that the author read at minimum Daniel J. Bernstein's design doco for daemontools and Laurent Bercot's design doco for s6. M. Bercot goes into some depth when it comes to the reasons why things are engineered as they are. Additionally, the first chapter in the nosh Guide enumerates some of the design fundamentals that underpin it, which daemontools-family systems generally share.