You only have to trawl through all the posts about airlines letting people down to put things in perspective (ever heard of a railway changing the date of your ticket and refusing refunds??).
Biggest problems of Eurostar via France is connections. Only a tiny number go to anywhere beyond Paris so you have to change to a French train. There is a chance that you could do this at Lille but otherwise it would be Paris - and unfortunately that means changing stations. That aside getting to Switzerland is now very easy as there are TGVs to Basel (from Paris Est) and Lausanne or Geneva (from Paris g.de Lyons). Rome is a lot further but could be done on an overnight train from Paris. Eurostar have just about sorted themselves out and offer proper through tickets, for years they pretended to be an airline without wings and wouldn't talk to other rail companies.
The connections at Brussels are easier as use all long distance trains use the same station, you can get to Switzerland but it takes longer. You can also reach Germany, especially the Rhine, in an easy day. Or the Netherlands, which has always had easy through ticketing.
Don't forget, unlike many airlines the train fare includes your luggage
but you have to carry it with you.
There are three main options for tickets in Britain, Eurostar themselves (may limit destinations they sell), Raileurope and their agents, Deutsche Bahn UK and their agents. Because of exchange rates and other variables they don't always charge the same prices.
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