FOSS platforms with closed clients are fine, because nobody has any unilateral ability to dictate the direction of the platform. You can always choose whichever client you want, so long as the underlying platform is FOSS.
Reddit and Twitter have proven that closed platforms aren’t compatible with FOSS clients because eventually, the closed platform will force out all competing clients to monetize every last bit of traffic it can.
And honestly, I have a higher degree of trust for Sync Apps LTD than I do for most others because they have historically been above-board in their dealings and have always been user-forward in everything they do. If that changes, then it changes. And so might my decision to support them. However, considering their entire business model is and has been dependent on the continued loyalty of their user base, I doubt they will make any significant changes that could affect their reputation.
And let’s be honest, where software is concerned, there’s a reason that all software isn’t FOSS, beyond just “greedy capitalist overlords”. The overwhelming majority of FOSS software is simply not as user-friendly or, in many cases, stable or even usable to people who don’t possess a significant degree of computer software and hardware literary. Most folks don’t have the know-how or time to go combing through dozens of conflicting and outright contradictory online resources trying to figure out exactly which .lib file is causing their software to crash on startup, or to learn how the hell to use github.