There are scam coins and alt wallets with malware inside. You can never 100% know what you are downloading.
Weaker network, compared to Bitcoin, it is almost nothing.
Algorithm and features not fully tested, so they are more prone to have unknown vulnerabilities.
Other algorithms like PoS have other known vulnerabilities, like some kind of 51% attack by having more than 50% of the coins
Topic: Bitcoin Core Developers say Altcoins are unsecure - page 2. (Read 1981 times)
For proof of work coins, the more people who are mining and the higher the hash rate, the more secure that network becomes. Bitcoin's hash rate dwarfs everything else out there at the moment. This means if some people have a sizable amount of hashing power and turned it towards a smaller coin, they could control more than 51% of that coin's network. Other coins that don't use proof of work may be considered more vulnerable because their networks aren't quite as well distributed, or possibly even entirely centralised, which means you have to trust who ever is running it.
Not all, but some alt coins are trojans or pump & dump.
The more popular a coin, the higher the price, the more people mine = more computing power distributed = more secure coin.
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