I've been using a macro for years to launch my startup apps, because I found that leaving it to the MacOS causes some apps to fail to launch. It's a fairly lengthy, interactive macro, and I've been using it for years without issue.
Over the last few weeks, it's started failing, but not consistently on the same task. I haven't updated the OS recently.
Generally speaking, does KM lose track if the macro has too many steps? Would I likely increase reliability if I broke this into multiple macros and ended each one with a launch action to the next?
Quite so, or if you do not want to do that for any reason, @anamorph, try to narrow the issue down enough that you can supply a test macro that demonstrates the problem.
If you have not already done so, be sure to work through the troubleshooting pages in the manual and in the wiki.
Have you got any other startup macros that might be interfering? Are there any "on application launch" macros sticking their oar in? You should be able to spot either by checking the Engine log (~/Library/Logs/Keyboard Maestro/Engine.log).
You can then either serialise or cancel the problem macros using semaphore locks.
Otherwise, it could be as simple as one or more app updates resulting in a different context after launch so a condition fails, something's taking longer than expected so there's a timeout, etc. Again, the Engine log should give you some clues. Various macros have been posted to the Forum that'll take you go directly to a macro's action in the Editor from the action's UUID in the log, which makes troubleshooting much easier, so grab one if you haven't already.