Leavers have been banding about the "we don't know what will happen' argument ever since they truly lost the economic argument. At this point, the economic argument is pretty much a slam dunk for the remain side. The question isn't whether leaving will be worse for us economically, but by how much.
Ignore everything said, just look at how the markets are reacting to the swings in voting intention polls. The pound saw it's biggest one-day rise in 8 years after the weekend shift towards remain. Whereas the value of the pound kept dropping last week as Leave was gaining ground. The value of the pound has been decreasing for the last year, ever since the referendum date was set.
The truth is that investors hate uncertainty. With good reason; why would you choose to invest in a time where it is unpredictable whether you can make a return on your investment? It's an entirely self-fulfilling prophesy - investors don't invest, causing a drop in the markets, which investment-holders then worry about and start selling. And these are only the effects from investors following polls and headlines. Particularly anxious investors sell following polls; most will start selling when we have left the EU. If we leave the EU, expect an immediate and sizeable drop in the pound.
As for trade... imagine you have a company where you have sourced your materials from a European nation for the last 10 years. Your expertise, your contacts, your knowledge of your suppliers all come from having spent years working with EU-based partners. The EU may or may not impose trade tariffs, and there may or may not be new avenues to source your materials from as the UK may or may not succeed in improving deals with other countries. But how can you run a business effectively with that uncertainty? A year from now, you have no idea where you will be sourcing your materials from and you have no idea who you will be marketing your products to. You may have to abandon a market you have been making in-roads in for years.
For example, alcohol. You have been importing a Belgian ale for 10 years. The locals love it, you're making a tidy profit off it. We leave the EU, it becomes less practical to order that ale, so you import from USA instead. Now you need to find a new supplier, find a way to import it, predict how fast the locals will take it up in order to know how much to order etc. The Belgian ale was steady; the States ale is unpredictable.
"Not knowing what will happen" is in itself a bad thing for business owners and investors. Which is why the short term picture looks fairly bleak for the UK if we leave.