Since inception the Bitcoin and MWC over the counter trading markets have thrived. Many find this way of acquiring to be far superior to exchanges because it can be done in larger size with less slippage and more discretely.
As Nick Szabo observed, "Trusted third parties are security holes." While we appreciate the exchanges that list and support MWC, nevertheless, a robust OTC market is very important for providing liquidity.
Dealing with trustworthy counter-parties with long standing reputations of successful trading is very important. There are plenty of dishonest fly-by-night scammers trying to steal your precious BTC and MWC.
An issue with a proof of work blockchain in its infancy is hash rate. As it claws its way out of the primordial ooze it may be vulnerable to block withholding and/or 51% attacks. These types of attacks are more annoyances than anything else. Binance Academy explains the types of disruption a 51% attack may cause and users ought to understand them.
The best defenses are to deal with trusted counter-parties who can always resend a transaction if it gets orphaned or reorged and to just patiently wait for more confirmations.