The phrase "almost too complicated" is correct and usable in written English.
You can use it to describe a situation, concept, or task that is very complex, to the point where it may be difficult to understand or manage.
Example: "The instructions for assembling the furniture were almost too complicated for me to follow."
Alternatives: "nearly overly complex" or "practically too intricate".
The whole thing is almost too complicated to function.
Besides, the diligence required for most buyers is almost too complicated.
But it also wins praise from independent tax experts for repairing what they call a body of law almost too complicated to comply with or to enforce.
With four candidates hovering somewhere in the vicinity of twenty per cent, the permutations of possible matchups and outcomes were almost too complicated to contemplate.
All of that was tied into gizmos Mr. Clough, Ms. Bensko and others in their office hid in the apartment — without telling the clients — in a way that is almost too complicated to explain.
The film sounds simple enough; but the various components seldom behave as they should, and there are roles for Tim Roth, Amanda Plummer, Eric Stoltz, and Rosanna Arquette that are almost too complicated to explain.
How about books that are so unique to the genre that they almost seem too complicated to adapt?
The holistic complexity of systems states that within the 7-level scale of system magnitudes, almost all systems are too complicated to be cognitively understood or mentally handled as a whole, except small systems or those that can be decomposed into a set of small systems according Eq. 3.2 in Table 4.
At first it seemed too complicated, almost impossible to get stockholders and corporations to divest from doing business in and with South Africa.
Almost everyone thinks the federal tax system is too complicated and in need of major overhaul.
In fact, it's almost impossible to fully describe the situation – it's too complicated, and too global.
