Tundra starts can be great, or awful, depending on what you make of it. For Russia, playing peaceful or waiting for late-game domination, it is amazing (faith, strategic resources, room for numerous small cities, deer, fish, only having 1 neighbor, etc). You couldn't ask for a better start bias (except maybe desert). For Sweden, it is much more iffy, since your great person generation requires you to actually work specialists, which you won't have enough food in the tundra to support. But, needing a military means you also can't use many food routes. So, you actually need to take out a neighbor and get some good land asap before you set up your great person city. But, taking out a neighbor before having friends makes everyone else hate you, which is bad for Sweden too. It's a mess. Sweden is uniquely ill suited for the tundra. Civs with no start bias would all do fine there.
The worst start bias is Plains. There's nothing inherently wrong with it, you just can't do anything about it. It is entirely inflexible. Forests and Jungles have grasslands beneath them; Deserts and Tundra have faith and natural protection on one side; Hills are near mountains. Plains gives you nothing to hang your hat on and no ability to grow your population. It's the opposite problem as Grasslands, and as usual, between food and hammers, food wins. Also, the Grasslands bias are for India (which needs to grow anyway) and Dutch (which needs the marshes nearby)... so they're well suited. The plains bias is for Mongolia and Poland, and you'd hear a lot more complaining if those civs weren't so strong. Poland needs plains because it's the only land that supports all three of the stable animals (horses, sheep, cattle), but note that this does leave them open to invasion from everywhere, hmmm, and is otherwise not particularly desirable. The Mongols need plains because they need horses, period (only available on grass, plains, and tundra). Grass has no hammers (and only when you need to attack at a specific time, do hammers become more important than food). Tundra has trees, which slow down horses. So, despite all three being historically accurate, the game went with Plains.
If you take a random no-start-bias civ, Plains would be the worst start for most of them (worse than Tundra/Desert/Jungle). However, of the civs with a start bias, Sweden-Tundra may be the most mismatched. The weirdest part is that it was a recent change brought on by BNW. Sweden was fine with no start bias before BNW. Now, it's unfairly crippled. For Sweden, I would change the start bias to "avoid jungle".
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