What are some of the problems of ZC

As the title says

It's free, it has a huge amount of functionality built in, it's open source so you can modify it to meet your needs, it's PCI compliant out of the box, it's got a great support community, there are plenty of mods available...

Oh wait those aren't problems. In my opinion it's one of the best platforms out there. I can't say if it will meet your needs and that it won't ever break but I'd recommend investigating if it will work for you.

Perhaps it would be helpful if you could state what you would like it to do?

The main problem in general is the customisation usually still requires some modifications to core code and subsequently the upgrading experience varies from tedious to nightmare depending on how well you have used the override and notifier system and documented your hacks.

Functionality hole#1: no stock control of variants (attributes). This requires add-ons.
There is a long-running bug(s) with rounding errors that requires a clean sweep, that I expect will be done in 1.6.
People may moan that "friendly urls" are not built-in like other carts, but that functionality is irrelevant for SEO and so way down on the ZC to do list (if on it at all).
Otherwise it all works as advertised, and if you find it doesn't, it is quickly fixed.

You need to try it out and check it does what you want, same as other carts.

Zen Cart is great, but out of the box it doesn't support SKUs. Thus when your stock situation gets more complicated, you're in a world of pain.

Zen Cart is great, but out of the box it doesn't support SKUs
If you mean a location-based product code (different codes for same product in different locations), ZC does not handle that.
A simple code per product: yes it does that = model.
A code per product-variant: no, it does not do that, as I implied above.
You need to use Stock by Attributes (can't comment on that, never used it) or Products' Options' Stock Manager (commercial, completely fills this hole).

i agree with steve about the inventory problems if your product has attributes. problem that one needs to address...

i appreciate everything that the ZC team does; with that said, i think ZC could be much better with coding once and only once. there is just far too much duplication of code. i know the team is addressing this issue for future releases, but it frequently comes up as in this new post:

https://www.zen-cart.com/showthread.php?222861-Placing-the-text-on-the-side-on-both-pages

different sets of code for displaying a category v displaying all products v displaying new products, etc. and this is just one example.

i think the overall structure could be a tad better to minimize this duplication, and again as steve said, a better way for handling overrides and custom coding so as not to make upgrades so much work for those sites that are heavily customized.

that said, it has a great community and it is very flexible as far as doing what you want your shopping cart to do.

just to add to new/potential users finding this thread...

Zen Cart is on the path to righteousness, the devs are as meticulous as befits a core business software,

the community/forum has no equal in terms of support for the DIY shop owner/part-time developer,

but the progress/release cycle as seen outside Github, is positively glacial...although I suspect this is deliberate, bearing in mind the major effort an upgrade entails for most shops for the reasons mentioned previously.

Upgrades for 1.5x are pretty much maintenance.

The upgrade to 1.6 will go a long way to remove code duplication and further prepare the path for easier customisation.

The unicorn that is/was 2.0 should be at least the equal anything in the market...but I expect that by the time that gets spotted in the wild there will only be two online shops in existence anyway, so we will all be retired.

New users need to understand, very clearly, that if they need any additional features there is a good chance that there will need to be some PHP code merging done unless it is something simple such as shipping, payment or some order total modules. So this cart may not seem as user friendly as many other "modern" carts that were developed in the last 5 - 10 years and have drop-in plugins, due to zen cart being based on legacy (very old) OSCommerce code base. This means that new users will most likely need to be willing to do some coding/learn how to do some relatively basic PHP coding or be willing to pay someone to do it for them.

That said, this is excellent software for those willing to invest some time learning the directory structure which is not always intuitive when starting out. I appreciate that it is still mostly procedural PHP instead of object oriented PHP so the files are easier to read and edit for non-experts such as myself.

I don't think I will ever switch to a different platform, I have tried a great many to see how they compare, and I stuck with zen cart because I found it easiest to customize to my needs. The community support here is pretty fantastic although I have noticed that there seem to be fewer people involved lately.