star trek - Why are all Federation starships similar in design?

All Federation starships I have seen have a very similar configuration. Is there any in-universe explanation for this?

I know this configuration comes from human ships like the NX-01, but why was it kept for the federation after it was formed? why not follow a Vulcan or Andorian design instead? Is there any inherent benefit of having a big round section and external engines?

I remember from an Enterprise episode that they emit some radiation that is dangerous, but there should be a way to shield it to allow a less fragile design.

Answer

In-Universe, the Federation has a standard design for their warp engines, and they use a technology for power and warp travel which is somewhat different than other species use. The necessities of their design compel them to build their ships along a standard model (saucer + nacelles).

Form follows function, in this case.

There is also a unification aspect: the Federation is composed of many species, some of which are radically different, physically and physiologically. Their ship design is part of the Federation shared culture, and serves to give all the races a cultural touchpoint. The origins of this go back all the way to the NX-01 and its sister ships.

Other races seem to use different technology (or at least a different method of nacelle design) for their propulsion. Romulans, for instance, somehow harness energy from micro black holes. Thus, other species designs follow their own culture, limited by their engineering constraints.

Edit:

Here's a page which includes some Star Trek starship design guidelines, including nacelle pairing (they MUST be in pairs), placement (must be visible from the front), and size. They are included in a site devoted to people designing new types (for fanfic, fan images, etc) but are purportedly from Gene himself.