We use a Cirix connection, along usernames/password and cryptocard
code, for our employees who are not able to come to the office. At
first it's only used for reading email. Now we want to place our
business administrationsystems online. In that way our co-workers at
home can do their job from their bedrooms (if they want). We are in
the business of asset management and the financial market.

My question is: How save is this Citrix environment, used as a WAN
business network, compared with the local LAN in our office?

I support a Citrix environment for about 700 people.  We have deployed
large parts of our environment via Citrix and I think the answer is
yes.  I use the Citrix Secure Gateway (CSG) as well as make sure that
my Citrix servers are sitting in a DMZ on the network.  This basically
gives me a encrypted connection to the client and with the network
segmented I can audit the network quickly.  I think its as safe as you
can expect for a reasonable cost.  I see the biggest point of failure
for this at the Citrix application login but it sounds like you have
taken steps to address that with the cryptocards.