I read some guide posts about custodial vs. non-custodial wallets but only a few about open vs. closed source. I often read them brought up by other users in the comments. I think many beginners don't have the slightest idea about the difference between closed and open source yet. I don't claim to be an expert on this but I will try to highlight that in this post based on what I understand so far.

Closed source: Since nobody else knows the codes except the developers, it is mainly them who can check the wallet for any bug or other vulnerabilities. I'm not sure how many are they but their number is surely fewer than the devs monitoring open source wallets. If a hacker finds a malware, it could probably take them longer time to fix that. The longer the time, the greater the risks to the funds of users.  

Another factor is you have to completely trust or at least highly trust the team. I think being a "trust-based" wallet alone is a potential red flag already and you should think carefully before using them. Why? The developers can do a lot of changes in the wallet. I can't remember who said this but here's an example, the developers can ask you to pass KYC verification before you can access your funds again. That sounds crazy and the chances of that happening are low but it is still a possibility.

Non-custodial mobile wallets used by many beginners today are probably unaware that they are closed source. Here are some of them:
- Coinomi
- Exodus
- Enjin
- Jaxx
- Trust Wallet (Android)


Open source: Huge advantage over closed source is there are more people who can check the codes for any bugs or malwares. The more eyes to inspect, the faster it is to find and fix any vulnerabilities. The wallet becomes more secure to store funds. There is also no need to put trust to a few developers and this idea truly supports decentralization too.

Example of non-custodial and open source mobile wallets:
- Electrum
- MyCelium
- Bread Wallet
- Trust Wallet (IOS)


I am sure there are similar or differing opinions from the pro users here and I would like to hear them.
Please share your thoughts, add more, or rectify if you see something inaccurate in my post.  


P.S.
I'm not promoting any of the above-mentioned wallets. If you want to use any of them, please do it at your own discretion.