Some Reformed people protest this evaluation, saying that the various Reformed denominations do indeed accept each others' legitimacy. But, as I've argued in many places (such as here and here), such a claim makes no sense in a presbyterian context. According to presbyterianism, the oneness of the church involves in its essence collegiality in its government--that is, churches which accept each others' legitimacy must function together in mutual submission to each other. But divided denominations are not in such submission to each other, by definition. Some protest that there is some connection between the denominations. For example, many of them join together in various non-binding Reformed organizations (like ICRC and NAPARC). The problem is that these organizations are non-binding. Thus, they do not represent an expression of presbyterial mutual submission between the denominations.
It is helpful here to contrast the situation of the Reformed denominations with the situation of the Eastern Orthodox churches--particularly in the United States. Due to a number of particular historical circumstances, there are many EO jurisdictions existing in the United States which overlap with each other. This bugs many EO, because it presents an appearance of disunity. For example, in the area where I live (northern Utah, near Salt Lake City), there are overlapping jurisdictions--including the Antiochian Archdiocese of North America and the Greek Orthodox Archdiocese of America. Ideally, distinct jurisdictions ought not to overlap. (If you want to get a better feel of what I'm talking about here in terms of what's going on with the EO church in America, check out this helpful article by EO Father Andrew Stephen Damick.) Does this mean that the EO church is in a similar state to the various Reformed denominations existing in the US, often overlapping each other by having presbytery boundaries overlap, etc.? No, it does not, and the difference highlights the disunity of the Reformed churches compared to the EO.
The EO, despite the messiness of their jurisdictional situation in the US, still recognize each others' legitimacy and authority, and this is manifest tangibly in various ways. For one thing, all the Orthodox churches in the US share the same faith, practice, and worship, unlike the various Reformed denominations. Also, the EO churches function in mutual submission to each other governmentally, unlike the Reformed churches. They accept the idea of larger, mutually-binding councils between themselves. One of the most interesting tangible manifestations of their governmental unity is the existence of a body called The Assembly of Canonical Orthodox Bishops of the United States of America. On the website of this body, you can find lists of churches that are in full communion with each other and are officially recognized as canonical--i.e. possessing de jure legitimacy. You'll find no such official list recognized by the various Reformed denominations. The Assembly is also working in concrete ways to end the jurisdictional confusion. Here are some quotations from the "About" page of their website (linked to just above):
The Assembly of Canonical Orthodox Bishops of the United States of America is one of thirteen bishops' assemblies that have been established in different geographical regions throughout the world. It is made up of all the active, canonical Orthodox bishops of the United States of America, of every jurisdiction.
The Assembly was established in accordance with the Decision of the 4th Pre-Conciliar Pan-Orthodox Conference, convoked in Chambésy, Switzerland, June 6-12, 2009, at which met representatives from all the universally-recognized autocephalous Orthodox churches. These representatives recognized substantial canonical "anomalies" in the organization and life of the Church in these regions, and realized that, though these anomalies had arisen from specific historical circumstances and pastoral needs, they nonetheless present a number of serious problems for the faithful; moreover, they give an appearance of disunity in the one holy Church. As such, these representatives unanimously agreed to the formation of the assemblies of bishops to heal, as quickly as possible, these anomalies. . . .
The purpose of the Assembly of Bishops of the United States of America is to preserve and contribute to the unity of the Orthodox Church by helping to further her spiritual, theological, ecclesiological, canonical, educational, missionary and philanthropic aims. To accomplish this, the Assembly has as its goals: i) the promotion and accomplishment of Church unity in the United States ii) the strengthening of the common pastoral ministry to all the Orthodox faithful of the region; and iii) a common witness by the Church to all those outside her. In addition, the Assembly has as an express goal iv) the organization of the Church in the United States in accordance with the ecclesiological and the canonical tradition of the Orthodox Church. . . .
Unlike SCOBA however, the Assembly is a transitional body. If it achieves its goal, it will make itself obsolete by developing a proposal for the canonical organization of the Church in the United States. This proposal will in turn be presented to the forthcoming Great and Holy Council, which will consist of all canonical Orthodox bishops throughout the world. Should this proposal be accepted, it is hoped that the Assembly of Bishops will then come to an end, ultimately to be succeeded by a governing Synod of a united Church in the United States.
The various Reformed denominations in the US, or throughout the world, could never have an organization like this, because the Reformed denominations are not unified in faith and practice, and they are unwilling to recognize any higher council outside of their own highest general assemblies. There is no Reformed "Great and Holy Council" coming up to which plans for organizing and systematizing the various denominational jurisdictions could be submitted. The EO are working to make it so that there will no longer be churches of the Greek Orthodox jurisdiction in the same locations as churches of the Antiochian or Russian Orthodox jurisdictions, etc. But the Reformed denominations will never submit to a situation where there will no longer be any overlapping Reformed denominational jurisdictions. The EO, despite some administrative confusion, are still one church, unified in faith and practice, with all the branches in mutual submission to each other. Not so with the Reformed denominations. So perhaps it is time for us to face up to that fact and stop pretending that we have more unity than we do. Then we will be in a better position to seek the full unity of Christ's de facto church on the earth.
For more, see the above links, and also check out this book.