[jpm_sd]

This is all true. The biggest deal breaker for any kind of serious work is being constrained to two layers, routing is so much harder that way!

Milled PCBs also have lousy surface quality which is incompatible with fine pitch parts.

[Junk_Collector]

Surface quality on milled PCBs can be pretty good but you have to get a bit spendy. Also, the copper FR4 likes to delaminate under stress and deforms easily if you use the wrong cutting parameters so you typically want to go for a higher quality board material. It's pretty common for high end RF boards to be milled.