Not measurement tools; propaganda tools
The "M-Lab" measurement tools aren't accurate. I tested their "NDT" tool on a 3 Mbps connection, with no NAT, and it reported a downstream throughput of 39 Kbps. It then froze up and didn't finish whatever other tests it was intended to perform.
Clearly not ready for prime time.
But of course, this doesn't matter. These "tools" aren't serious measurement tools; no one who is in the business of actually providing broadband service (in other words, no one who really understood how to build such a tool) was involved in writing them. They are, rather, propaganda tools put forth by advocates of Internet regulation (aka "network neutrality"). They're designed to find fault with broadband connections, even if they actually work quite well, so that their authors can yell for regulation of the Internet.