As markets hit new highs, it's natural to wonder what might end the party, and pundits at the Inside ETFs conference were asked just that question: what keeps them up at night?

Dennis Gartman, Editor of The Gartman Letter, is worried by the risk of a geopolitical crisis. He cited a new outbreak of fighting in Kashmir where of course both India and Pakistan have nuclear weapons. He's also concerned that China's increasing aggression in the South China Sea could trigger a conflict.

There's also a lot of concern about what might happen with inflation. Lori Heinel, Deputy Global Chief Investment Officer at State Street, thinks inflation will probably only rise modestly in the US in the near future, but there's a risk of a more aggressive rise and that's her top worry. Modest rises in inflation are normally good for equity markets, but a more substantial rise would probably be damaging thanks to interest rate rises following the pick up in prices.

Richard Bernstein of Richard Bernstein Advisors thinks markets are overly sanguine about inflation. He highlighted fact that US consumer inflation peaked at 5.5% in the last cycle in 2008. He said: 'I don't think most portfolios are geared for 3% inflation, let alone 5.5%.'

The other big concern is trade. Kurt Reiman, BlackRock's chief investment strategist for Canada, fears that a US withdrawal from NAFTA wouldn't just damage the North American economy. It would also give a negative broader signal on global trade.

Bonds

The future for bonds were also very much on the agenda. Bernstien is very bearish on bonds.

'If you're a bondholder now, you're arguing that the business cycle is dead, that's ridiculous...bond investors are playing ostrich. Inflation is going up and inflation expectations are going up too.'

Gartman reckons investors shouldn't be invested in bonds at all, 'not even marginally.'

Heinel was more positive. She thinks that interest rates won't rise that quickly and when it comes to the bond market she argued 'there's a lot of money on the sides waiting to come in when interest rates rise.' So that extra cash will stop bond prices falling too fast.

That said, she's still underweight in bonds and overweight in equities.

Equities

Even though all the pundits were happy to talk about the scenarios that keep them awake at night, in broad terms, they're still fairly positive on equities. Heinel reckons '2018 is shaping up to be a pretty good year for investors.' She expects the global synchronous economic growth we've seen to continue - she pointed out that only six countreis aren't growing at the moment, and they're mostly affected by war.

Reiman thinks 'the market isn't yet at a point where it's irrational… although you should perhaps lighten up on some sectors that are seen as bond proxies.' US-based investors should also focus outside the US and 'particularly emerging markets where there is still room to run.'

Commodities

Commodities were only raised by one panellist - Dennis Gartman. He's mildly bearish on energy thanks to the potential of increased production thanks to fracking. But he's positive on all other commodities and expects them to rise as bonds fall.