There are some great travelling rides in the UK that can't be found in theme parks over here, and if you're a fan of spin rides then fairs are generally your best option.
My issue with travelling rides is the variation in price and ride quality you get due between (and even within) fairs. At a busy fair you'll pay a lot of money for a very short cycle, but when it's quiet you'll pay less and can get far more generous cycles than most theme park flats offer.
Sadly UK fairs are a poor relation of their European counterparts, the largest Dutch and German fairs host coasters that rival those found in many theme parks, meanwhile we're still left with an infestation of spinning mice.
I have no concerns with fair ride safely - the rides are their owner's livelihoods so it's in their interest to ensure they're safe and operational. Due to the fact they're dismantled and reassembled every week it's much easier to inspect the parts than static rides, so mechanical failures are very unlikely.