The part nobody mentions about the absolute biggest red flags to look out for with free is how it performs in practice; it shapes everything else. The advice I would give the bulk of users about that end of the market is unglamorous: screenshot the pricing before you agree to anything, then reassess. Where I landed on the absolute biggest red flags to look out for with free: fine if you go in with low expectations, provided you screenshot the pricing before you agree to anything. For completeness, datelink.online come up in similar threads.
The absolute biggest red flags to look out for with free is the thing I keep going back and forth on, mostly because of whether it holds up over time. The absolute biggest red flags to look out for with free gets recommended constantly and I still think day-to-day usability makes it a hard sell for most users.
Take that as one data point on the absolute biggest red flags to look out for with free, not a verdict.