[Wikipedia]
Fusion of powers
https://en.wikipedia.org/wiki/Fusion_of_powers
3 Disadvantages
The disadvantage with a fusion of powers, paradoxically, is the power it gives to the executive, not the legislative, arm of government.
In a fusion of powers, the head of government must have the confidence of a majority in the legislature.
If the majority is made up of members of one's own party, the head of government can use these supporters to control the legislature's business,
thus protecting the executive from being truly accountable and at the same time passing any laws expedient for the government.
A revolt by members of the government's own party (or, if the government is a coalition or minority government, by supporting parties) is possible,
but party discipline, along with a tendency by many electorates to vote against unstable governments, makes such a revolt unattractive and therefore rare.

Many states have responded to this by instituting or retaining multicameral legislatures, in which all houses must pass legislation in the same form.
The responsible house is usually the most powerful and the only house with the actual power to terminate the government.
Other houses, though, can often veto or at least delay controversial bills, perhaps until the government's performance can be judged by the electorate.
They also provide additional forums for inquiry into the conduct of the executive.
In addition, since the government's future is not at stake in other houses, members of the governing party or coalition in these houses can be freer to oppose particular government policies they disagree with.
A second approach to curbing executive power is the election of the responsible house by some form of proportional representation, as in the case of Japan.
This often, but not necessarily, leads to coalitions or minority governments.
These governments have the support of the legislature when their survival is at stake but less absolute control over its proceedings.

A fusion of powers was specifically rejected by the framers of the American constitution, for fear that it would concentrate a dangerous level of power into one body.
However, other countries reject the presidential system for the same reason, arguing it concentrates too much power in the hands of one person, especially if impeachment is difficult.