As SNR Denton, Salans and Canada's Fraser Milner Casgrain (FMC) prepare for their $1.3bn (£817m) merger, the scale of the proposed venture is unquestionable. With more than 2,500 lawyers working across 52 countries, the deal would put the combined business comfortably within the top 10 largest firms by headcount worldwide and just outside by revenue – admittedly a long way behind Norton Rose Fulbright, but still impressive.

But Dentons' management shouldn't expect much of a honeymoon as bigger does not always equal better and, in this case, the firm will have a lot to prove after Sonnenschein Nath & Rosenthal and Denton Wilde Sapte badly underperformed on initial hopes following their 2010 tie-up.

In fairness, the Canadian side of the equation appears to add up. FMC's oil and gas focus sits well with SNR Denton's practice and the deal gifts the combined firm a credible presence in a coveted stable market. There is also something distinctive to the union as the first between large US and Canadian practices.

In contrast, Salans' contribution looks less substantive, offering a host of offices and strengths across central and eastern Europe and a solid French hub, but little in London. These are not qualities to dismiss for a firm with a heavy tilt towards energy and infrastructure, but neither do they look entirely compelling given the pricing pressures and uncertain prospects in the CIS region.

What this fallen member of UK legal royalty needs is more productive partners in practices such as corporate, banking and disputes. While firms should not get hung up on profits or even short-term revenue growth, SNR Denton's UK performance lags its peers by a margin that makes it hard to retain and attract effective operators. Legacy Sonnenschein held up better in relative terms, but has still been an erratic performer in recent years.

As such, this tie-up will give the new firm a chance to reposition itself globally. But it will need a couple of years' credible post-merger performance under its belt. Realistically, given the mixed record of three of the constituent members, the assertion that the tie-up will make it immediately more attractive to laterals in the UK and Europe is surely over-optimistic.

The reason it will take time to win over a sceptical industry and potential recruits comes down to a troubled brand, which brings up the decision to use the Dentons banner. It's a proud yet somewhat tarnished name now, but then Elliott Portnoy was reputed to have seen the Sonnenschein brand as of limited value. Perhaps the best that can be said for the new firm is that it isn't complacent – the restless Portnoy can be described as anything but. That self-critical approach will stand the firm in good stead because it will take much more than new-look Dentons' expanded scale to make it a genuine global challenger.