* The pricing tiers and included features by tier penalizes you in frustrating ways. The base plan is a reasonable $6/user/m, but if you want to use ACLs to control anything in a workable way, it jumps 3x to $18/u/m. Better solutions are available for that kind of money, and I shudder to imagine what the next tier ('call us') costs.
* Subnet routing broke on Ubuntu (maybe other distros) recently, and there were no alerts, communication from TS, or TS tools to pinpoint/figure out what was going on. I stumbled on a solution (install subnet router on a Windows box), and from there I searched and found others with that issue. Lost half a day in emergency mode over that!
* Better tooling to determine why it's falling back to DERP instead of direct for remote clients. DERP relays should be an absolute last resort to provide connectivity for Business-plan-level customers (very slow), and the way TS works just assumes any connectivity is fine.
Overall, the simplicity and abstraction of complex VPN networking is wonderful, but if you have issues or advanced needs, you are immediately thrust into the low-level UDP/NAT/STUN world you were trying to avoid. At that point, you're better off using a traditional VPN (WG, OpenVPN, or heaven forbid, IPSec), because it ends up being more straightforward (not easier) without the abstractions and easy-button stuff.