Yes there are pitfalls with zero commission brokers. The US have more brokers and competition is fierce so it is much easier to find reputable zero commission brokers, as the trend has been industry wide with the bigger brokers such as TD Ameritrade have gone the route as Robin Hood. But here in the UK it's a different story as it's not that available. It becomes much harder if you are in the UK and want to trade US stocks daily.
I know that brokers such as Freetrade (app), Trading 212 and eTorro exists but each presents a problem if you're willing to raise yourself from being a novice to trading frequently. Many don't allow you to set a stop loss in the same buy order. You'll have to set it up later which can be difficult if you're scalping with precision. In all it depends what type of trader you are, buy and hold, longer term, swing, or short, day trader, etc
My problem is the hidden cost. For example with eTorro, Apple was offered at $122 on their platform some time ago but if you checked market live data was $120, a difference of $2. If I bought 100 shares of AAPL I would have paid $200 in commission just for buying the shares, (most retail traders won't realise this the spread) which would be detrimental overall and a broker such as HL that charges £11.95 + 1% FX fee would seem at first much cheaper. The problem with HL is that if you are a lower volume retail trader, and have a small account, the broker fees takes a lot out of your profit. HL is more geared at high volume traders and with a small account you can't set a stop loss to protect yourself. You can only buy or sell if you already bought it before, in that you can't short (due to the PDT rule and capital req). Hence why I am trying to find another cheaper broker. I'm caught in the middle of a novice and an experience trader with a large account not having that §25.000. I'm trying to build my account up organically.
I've heard about zero commission broker Trading 212 but not too sure, whether they are alright or not. It's the Bulgarian connection and the terms and conditions that they can move your money without telling you I read that worry me. Yes they are FCA regulated but would you trust your savings with them? Maybe, maybe not.
I remember, but do correct me if I am wrong, IG charges £8 for the first 2 transactions and commission free afterwards, though they charge FX fee of 0.5%. The equation becomes even more difficult if you're trying to find a broker that supports ISA Stocks and Shares. I'm not interested in dealing with CGT - the calculation of each transaction, date, § and £ price and FX rate to send to HMRC, is just way above my daily administrative duties and I dislike Excels. Also my finances are not at the position to allow me to hire a PA to do these meagre calculations. If someone made this easier to track, I would just go for it.