Union-State financial relations in India are a critical aspect of its federal structure, often characterized by inherent tensions and controversies. These relations are governed by constitutional provisions (Articles 268-281) and the recommendations of the Finance Commission. The introduction of the 'One Nation – One Tax' policy, primarily through the Goods and Services Tax (GST), has added new dimensions to these controversies.

General Areas of Controversy in Union-State Financial Relations:

1. Resource Allocation and Vertical Imbalance: States often argue that the Union government controls a disproportionately large share of revenue sources, leading to a 'vertical imbalance' where states have more expenditure responsibilities but fewer revenue-generating powers. They frequently demand a larger share in the divisible pool of central taxes.
2. Centrally Sponsored Schemes (CSS): States often express concerns over the proliferation of CSS, which come with specific conditions and matching grant requirements. They argue that these schemes reduce their fiscal autonomy, force them to prioritize central objectives over local needs, and strain their budgets.
3. Discretionary Grants: States prefer statutory transfers (recommended by the Finance Commission) over discretionary grants (like those from the NITI Aayog), as the latter can be perceived as arbitrary or politically motivated, lacking transparency and predictability.
4. Borrowing Powers: States' borrowing powers are subject to central government approval, especially if they are indebted to the Centre. This restriction is often viewed by states as an infringement on their fiscal autonomy.
5. Cesses and Surcharges: The Union government's increasing reliance on cesses and surcharges, which are not part of the divisible pool of taxes shared with states, reduces the overall share of states in central revenues, leading to resentment.

Controversies in the Context of 'One Nation – One Tax' (GST):

1. Loss of Fiscal Autonomy: The most significant controversy is the perceived loss of fiscal autonomy for states. Before GST, states had the power to levy various indirect taxes (VAT, entertainment tax, luxury tax, etc.). With GST, these powers were subsumed, and states now have limited independent taxation powers, making them more reliant on the GST Council's decisions and central transfers.
2. Compensation Mechanism and Revenue Shortfall: While a compensation mechanism was put in place for five years (until June 2022) to cover revenue shortfalls, states have consistently raised concerns about the adequacy and timely release of compensation. Many states experienced significant revenue shortfalls post-GST implementation, leading to increased dependence on the Centre.
3. GST Council's Functioning: The GST Council, designed as a cooperative federal body, has faced criticism regarding its decision-making process. States, especially smaller ones, sometimes feel their concerns are not adequately addressed, and the voting mechanism (Centre having one-third vote share, states two-thirds, and a three-fourths majority required for decisions) can lead to an imbalance of power.
4. Exclusion of Key Revenue Earners: The continued exclusion of high-revenue items like petroleum products, alcohol for human consumption, and electricity from GST means states still rely on these for significant revenue, creating a fragmented tax system and limiting the 'one nation, one tax' ideal.
5. Compliance Burden and Impact on Businesses: While GST aimed to simplify taxes, its initial implementation brought significant compliance challenges for businesses, particularly SMEs, impacting economic activity and, consequently, state revenues.

In conclusion, while GST aimed to streamline the indirect tax regime and foster economic integration, it has undeniably reshaped Union-State financial relations, introducing new points of contention, particularly concerning states' fiscal autonomy, revenue predictability, and the dynamics of cooperative federalism within the GST Council. Addressing these issues requires continuous dialogue, greater transparency, and a commitment to genuine fiscal federalism.