pgloader copes gracefully with errors. The COPY command loads all rows in a single transaction, so only a single error is enough to abort the load. pgloader breaks down an input file into reasonably sized chunks, and loads them piece by piece. If some rows in a chunk cause errors, then pgloader will split it iteratively until it loads all the good rows and skips all the bad rows, which are then saved in a separate "rejects" file for later inspection. This behavior is very convenient if you have large data files with a small percentage of bad rows; for instance, you can edit the rejects, fix them, and finally, load them with another pgloader run.
Versions 2.x of pgloader were written in Python and connected to PostgreSQL through the standard Python client interface. Version 3.x is written in Common Lisp. Yes, pgloader is less efficient than loading data files using a COPY command, but running a COPY command has many more restrictions: the file has to be in the right place on the server, has to be in the right format, and must be unlikely to throw errors on loading. pgloader has additional overhead, but it also has the ability to load data using multiple parallel threads, so it can be faster to use as well. pgloader's ability to reformat the data via user-defined functions is often essential; straight COPY is just too simple.
pgloader also allows loading from fixed-width files, which COPY does not.