I believe you are know that this software is released by the Qualiteam, the X-cart is releasing by this company, so you should read the X-cart article firstly. The problems are common and I will talk about the difference only.
The Litecommerce software is build on clases and I think that the structure is perfect. It is allow to make any modules without touching the core or default modules. The original mechanism of the classes extensions is preset, you have to read about it in the documentation before development.
The great problem of the Litecommerce is speed. With the classes you are receiving the a great number of the SQL queries to the database and some additional steps to receive data. For example in any usual software some query is executing and you are receiving the array with data, here you are create the object firstly, then query(s) are executed to fill the object and then you are receiving the result. If you try to find a few products, instead of one query, there are a lot of it - at least one for each product.
So there are a lot of queries. Each Litecommerce query is very fast - it is going by the primary key. The mysql cluster (from 4 server) allow to execute about 100.000 queries per second. If there are 500 (up to 2000 for customized and filled by modules) queries per page, we are receiving 50-200 generated pages per second. It is good result, but we need the cluster. For example for x-art or any other software it is not possible speed on the same cluster, because each query is much “harder” in these softwares.
Summary, the special knowlenge is required for the development and the great server is required to run. But the Litecommerce is positioned as a “small business” software. How much small businesses can buy 4 servers for cluster? On one server the Litecommerce is not good - it simply floated and it is good if you will receive one page per second. It is not a problem of the software - because there are the solution, it is problem of the Litecommerce management team. The Litecommerce is the software for big business.
PS: I will prepare the addition article about the Litecommerce speed soon, it will show what is possible to make.