Stats between 2 tools will almost never match 1-to-1. (If they do match perfectly that’s an issue in and of itself but that’s for another post )
Keeping the above in mind, it really comes down to how big of a percentage difference you are seeing between GA and Unbounce.
Anything up to 5% is usually acceptable and not worth the trouble to hunt down and figure out why the numbers don’t add up.
Between 5-10% you have to be concerned but again, it depends on dozens of factors.
Mismatch above 10% should most definitely be investigated.
One of the most common issues when trying to match up numbers between different reporting tools is that you are not really comparing the same things (ex. visitors vs. sessions).
Another common issue is the reporting window and the “freshness” of the data.
GA, although improved in the last few years, still lags in terms of updating the data. It can take anywhere from 4h to 48h for the data to be fully updated.
It’s also important to note here that the timezone used for reporting can also play a role in the numbers you are seeing. High traffic pages can see a serious discrepancy simply due to the reporting window.
At the end of the day it might be simply an issue of misconfiguration of GA/GTM.
Since you are the only one with access to your Unbounce account and GA/GTM, you’ll have to decide how big of a difference you are willing to tolerate and what makes sense for your particular business.
It might be worth it to hire someone to audit your GA/GTM set up as a second pair of eyes that might spot some irregularities.
Finally, always pick one “source of truth” and use it for reporting purposes.
Over the years, working with hundreds of clients, we’ve found that using a well set up GA for reporting can save you a lot of trouble and headaches down the road.