Yes, the RSL (radio signalling link on abis) can be congested. This is very bad for QoS because it will slow down all main procedures on SDCCH (call estab, SMS, LU). I would say that due to “cumulative” effect of the congestion (snowball), the procedures could even fail.
To avoid RSL congestion, you must monitor the counter called “LapD Congestion”. On which system are you working ? ALU ?