Rob,

I would say offline businesses that have a high barrier to entry into the market before you can validate that you have a viable idea. This could be something like a medical device.

Entering a market with a lot of competition (e.g. pizza, clothing, creative agency) can be a real challenge if you don't have a noticeable and meaningful value proposition that clearly gives you an advantage against incumbent competitors.

Also starting any business when trying to only beat your competition on price (it is not a sustainable differentiator). You'll probably get involved in a race to the bottom (a.k.a. no profit margin) so you'll likely not be in business very long. This is especially true when long time businesses have much greater scale or profitability that they can sacrifice in the short term to run you out of business (because their profit margin can sustain a hit for an extended amount of time).

There are many other problematic businesses to start 'offline' but I think those are the largest problem areas.