No, they aren't less secure. I have been using core, but at some point it became a pain to use. Even if I remembered to turn it on once a day it needed 10 min to synchronize before I was able to send a transaction, but it gets better. One time my PC froze and I had to reset it and the whole database of core got corrupt and it took days to download. No more! Electrum is much better.
I believe you can't say lightweight wallets (probably he means SPV wallets) are as safe as full node clients.
With a lightweight wallet, you are ultimately trusting that the SPV servers are legit and not connecting to some sort of compromising third party spying on your transactions and whatnot.
With a full node, you are protected against any of that because you do do the validation yourself, which is what you want to do and the whole point of crypto (which is why big blocks are stupid btw)
Other than that, it depends on how safe you can keep your computer.