My main gripe with such a means to produce reviews are the following.
1 - You're in a controlled environnement, meaning the dev controls what you get to play to some extend, and how you play it. If they don't want you to experience the online mode, they make it unavialable, and so on. It can give the impression that they are filtering what gamers get to know before release, especialyl considering reviewers were pretty much not allowed to talk about anything that was not officially announced. In the end, it makes it more of an "Impression" than a "Review", as to me a review is supposed to be able to dissect a game as a whole, not just what it is convinent to discuss. Binge experiencing something in a controlled environnement is not the best for objectivity, basically.
2 - You're surrounded with Product Placement, memorabilia, posters from the game in question. You're also surrounded by the guys that made the game, making it harder to be critical. It's always harder to tell someone that what he spent the last five years doing kinda sucks. In sense, it's closer to being called for a playtest... You feel like you're "in the zone" with the devs and in a sense, you might let your guard down and forget you're a reviewer. This breaks neutrality. It would also feel somewhat wrong to play the game in front of the devs for five days, then write a review slamming the game for being crap.
3 - I'm not going to say it's bribing journalists, but you can be sure they get preferrential treatment. I'm guessing the trip to the boot camp is paid expenses, so is the hotel during their stay. They probably get meals, snacks and merch thrown at them, plus they get the opportunity to play the game days before everyone else and meet the creators. In a sense, it feels like VIP treatment, and it's bound to influence review scores in some way.
4 - Another problem is that it kinda controls who gets to review the game. The publisher hand picks the review sites who "deserve" to play the game early on, based on popularity, affinity and other corporate concerns. In a sense, the Publisher decides who gets the revenue ad from early clicks for reviews, too : By giving early access to a select few websites, they ensure said websites get more site traffic from their reviews, and ensure smaller, less known websites are left in the dark in terms of review money. Finally, this means outspoken folks like Jim Sterling , Yathzee and Angry Joe are filtered out of the initial batch of review, wheareas people that praise metal gear like Yong Yea get to play it early because it will generate sales. By the time the more critical reviewers get the game, their voices are buried by the initial batch of overly positive reviews. Again, review copies have the same problem, but the publisher can send more since it's just a plastic disk being shipped, not an all out paid expense trip they must host for journalists, meaning it's more democratic and less elitist.
5 - These type of review events probably tend to normalise reviews too. In a sense, if everyone is playing the game at their offices alone, they will all produce a different, mostly personal and unique opinion. But in such a setting, everyone's opinions have a chance to merge together and overlap, as the reviewers are constantly discussing the game. Peer pressure is strong, and if you realise all your fellow reviewers are going to give the game a ten, are you really going to go on a limb and give the game a 7 ?
6 - Last but not least, there is what I would call the "future opportunities" card at play here. In a sense, as a review website, you're getting the preferrential treatment now, but if you were to produce a steaming pile of hatred as a review, you can pretty much be guaranteed you would not be reinvited for the next similar event in the future. In the end, it becomes a situation where, if you were to absolutely hate the game, it would be detrimental to complain about it in the review and give it an average score, because it would make you look ungrateful and would remove you from the publisher's list of "publications that deserve to play this game early". In the end, you would lose ad revenue on the next game since you would be late to the party with your review. Just like review copie do to a lesser extent, this type of behaviour can influence review sales, as reviewers do not want to get blacklisted and lose revenue.