This has been one of the best things that have been applied within OBS, and I've made this account just recently to prove how good it is.. however, there's one minor issue that does prevent it from being a perfect 5 star rating.
The issue I tend to have is that sometimes the numbers will sometimes randomly skip, so if its 30 seconds for example it'll go smoothly and then skip it by 2 seconds at a time. For most folks its probably not a big deal, but if someone is doing a sub-marathon on twitch where every second counts or a speedrun.. that can be a massive issue for tracking the numbers!
I hope its just only my setup interrupting it, but please let me know if its just a overlook in the settings or its just OBS being OBS haha
The issue you're having is a known issue and its to do with the timer taking longer than a second to update the time which can happen the computer is being under load under certain conditions. I have tried to combat this for timers where you set the days, hours, minutes, seconds (Period Timer) with an option in the settings called "Steady Second Increment". What this does is that after every cycle of the timer no matter how much time it has taken it will increment the time by 1 second. This gives a smooth timer change every time but is less accurate. Hopefully this is helpful for your use case!