I can't count the times I've had to debug cron scripts by copying the cron command line and running it through sudo because the script ran into an edge case, only to find out I didn't have the exact cron environment so manual runs didn't trigger the bug.

I much prefer the systemd solution. As an added bonus, systemd services allow for easy configuration of things like sandboxes and resource limits that would need to be hacked into a giant concatenated command line in cron, presumably without comments even.

You do when testing it!

With cron this is troublesome, because cron uses an environment different from what you get from the commandline. So things can work from the shell and not from cron.

So to really check that cron works you have to add a line to be called in 5 minutes, then check your mail to see what happened.

With systemd though, you can launch the service at any time and see what it does without all that fiddling.

I'm constantly running the service ran by timer manually, mainly for debugging.

You might never run the service outside of a timer, but you may have multiple timers reference the same service.