The only issue I can see is the possibility that the PCs may get lost in the bad hexes (given an 8:1 ratio, unless they know exactly where the good hexes are, this seems quite possible).
As such, what about allowing the PCs to seek out relative safe zones inside bad hexes, with a fairly high Survival check? High enough DC that it's not guaranteed even with expertise, making good hexes the ideal places to rest but allowing the possibility even in bad hexes assuming great need. You could even make a low enough check trigger an extra random encounter (ie, you found a promising looking cave but the residents of that cave aren't happy to see you) so that this isn't a go-to move for players.
It's something we do in one of the campaigns I am playing in. The DM rolls a lot of random encounter checks for each day of travel (I think it's 3 traveling checks, 2 resting checks, and one check per short rest), although there is a secondary check to see the type of encounter so it they aren't necessarily all hostile. It's been absolutely brutal some days, but he does allow us to mitigate random encounters with either successful Stealth checks (while traveling) or Survival checks (to find a safe place to camp). A good check reduces the chances of an encounter while a great check eliminates the check entirely.
He also doesn't necessarily force combat. We were once waylaid by what I think was a Degenerate Titan from Tome of Beasts, who basically demanded our stuff. My friend had recently been adopted by an old frost giant (long, complicated story but suffice it to say that he had to overcome 3 nearly impossible challenges to be adopted into the frost giant's family). Anyway, as a "giant" my friend convinced the titan to allow him a rite of trial. The titan would recognize his gianthood and allow us to pass unhindered if he could drink an entire cask of whiskey. Thankfully, the character is a bit of an alcoholic and was able to pass the checks required, despite that the odds were quite stacked against him. I think we also hit him with a subtle spell Protection from Poison beforehand. Not only did we avoid a potential TPK, but we also made a friend and learned a good deal about the secret history of the giants.