Hello Martin
The auto-summary feature in BGP is used to automatically summarize routes at the classful network boundary, causing BGP to advertise the summarized routes of its own networks to its neighbors. The purpose of this command is to simplify the routing table by reducing the number of entries. It is useful in situations where you have very large routing tables and you want to reduce their size.
This of course has advantages and disadvantages, and you must weigh them carefully before using this command.
The advantages include a simplification of the routing table, reducing its size, improving efficiency, and simplifying management.
Disadvantages include a loss of granularity as you mention, and the potential for unintended route advertisements, especially in networks with discontiguous subnets.
So should you enable autosummary? Well, if you can avoid it, do so. Use it only if routing tables get too large and CPU and memory of BGP routers are being overly taxed.
Regarding the use of route maps, they are indeed a method to control and fine-tune what is advertised. Route maps can be used in conjunction with auto-summary to ensure that only the desired summaries are advertised, mitigating some of the risks.
I hope this has been helpful!
Laz