Here are the main concerns…

Here are the main concerns and downsides with Ontario's proposals to streamline service area and fill rate changes at waste disposal sites:
Reduced Environmental Oversight

The core risk is that loosening approval requirements trades rigorous scrutiny for speed. Under the current framework, a change to a waste disposal site that would expand the geographic area from which the site receives waste requires a full Environmental Assessment if it includes any area not previously authorized — and the same applies to fill rate increases that exceed previously authorized levels.
(Environmental Registry of Ontario) Streamlining these triggers means some changes could proceed without that scrutiny.
Host Community Impacts and Loss of Municipal Voice
Expanding a service area means more waste — often from distant cities — flowing into rural or smaller communities. Local governments simply don't want the environmental risks of waste disposal, particularly when the garbage comes from elsewhere. (National Observer) Communities like Dresden, Ontario have already experienced this tension firsthand.

Concerns raised in those cases have included potential impacts to species at risk, groundwater, surface water, increased truck traffic, odour, and other impacts to local communities. (Environmental Registry of Ontario)
Faster Landfill Consumption / Shorter Site Lifespans
Allowing fill rates to increase without a full review means existing landfill airspace gets used up sooner.

Ontario has committed to ensuring proposals for new landfills or landfill expansions include a rigorous review of the need to avoid over-supply of landfill capacity and the service area in relation to greenhouse gas emissions and environmental protection.

(Ontario) Streamlining fill rate changes bypasses that check, potentially creating new capacity shortfalls faster.

Undermining the Waste Hierarchy / Diversion Incentives Making it easier to accept more waste at higher rates reduces pressure on waste generators to divert and reduce.

Pressure on limited waste disposal capacity is already being exacerbated by the provincial government's own diversion goals (AMO) , and removing friction from disposal approvals can dampen momentum toward Ontario's diversion targets.

Audit-Based Compliance Is Weaker Than Pre-Approval Review Under the self-registration model, the Ministry does not actively review registrations at first instance — registrations are only audited periodically to verify compliance.

(Environmental Registry of Ontario)
This means problems may only be caught after operations are already underway, rather than being prevented upfront.
Cumulative and Long-Distance Impacts
Expanding service areas increases truck traffic, noise, and emissions across a much wider geographic footprint — impacts that a localized Environmental Assessment would normally capture but a streamlined process may not adequately address.

In short, the tradeoff is regulatory speed and cost savings for industry vs. environmental protection, community control and safety and long-term waste reduction goals.
Critics generally argue the balance tips too far toward operator convenience.