you can check google reviews or 12inch.com forum (in french only sorry) where there is a full topic about them, it's mainly about too high prices, changes of price before and after preorder, great lack of communication or rude communication, and lots of problems in case of damage or lost parcel...
Their prices are interesting on some brands, that's true (damtoys, artfigures...), curiously totally insane on some others (threeA, threezero, hot toys generally). Besides that they are one of the very rare site to ask for a paying membership (30€/year to get 10% discount).
From my experience I always avoided them after reading many threads of replacement problems, of unanswered mails for months, cancelled preorders without notification... But it's true people tend to be vocal and loud when things go wrong much more than when things go fine.
On the brighter side they were also well known 6 or 7 years ago for being one of the last stores accepting check payments even on international orders (they are in Belgium)
I agree with you that European stores tend to have higher prices because of taxes, mainly VAT, but the first and usual reason is because they don't buy enough quantity or don't declare it as business goods to be resold (they would pay an import tax this way, but no VAT, which is covered by the final customer, at least in france). Therefore they pay the same price as you and me if we were ordering from an american site or else, except they add their margin to this price, and then add VAT (that's where most of them make an additional margin since they calculate the VAT ratio on a price already including a VAT amount)