http://www.pokemonshowdown.com/replay/ou12959326
I feel like this replay showcases a lot of the ideas presented in this thread.
1. Overloading: My opponent had THREE lando "counters." Yet, even without ttar's pursuit support, they were worn down somewhat easily, at least by the end of the battle. This match really did change my mind about how easily lando's counters could be dismantled. But even so, that may all go to waste because....
2. Coverage Sucks: Lando's options for coverage moves are focus blast and hp ice. While this is great in terms of typing, focus blast has 70% accuracy... and even when lando's counters were all very sufficiently weakened, this halted the sweep. Not only that, but hp ice is 70bp unstabbed... and it causes lo recoil. So, from what I've gathered, although it is somewhat easy to weaken his counters, lando's coverage can still make it difficult to sweep properly. Last but not least...
3. Team support isn't /that/ important: While the whole lando+keldeo+ttar core is nice, it is somewhat unnecesary and doesn't work as planned in practice. On paper, it seems like ttar could come in and pursuit both of their counters, allowing either to sweep. But in reality, many of his/their common checks+counters (rotom, gyarados, etc.) aren't even pursuit weak and can harm ttar, and even the ones that are (latis) can easily play around the pursuit, not to mention that pursuit is huge setup bait for something like lucario (just an example, as he can then ko lando or w/e).
Despite this, though, in practice lando truly doesn't actually need the support that in paper seems so vital. Although teammates play a huge rule, it isn't as direct as everyone is making it out to be. Based off of that it is clear that momentum is vital when using lando. Finding safe switch ins to either set up or just fire off super strong attacks is absolutely key.
tl;dr: I (now) think lando does derserve a test, but I don't think he is broken due to the huge flaws (coverage) that inhibit his sweep.